Adding upstream version 1:115.7.0.upstream/1%115.7.0upstream

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

16 files changed, 2673 insertions, 0 deletions

diff --git a/intl/l10n/docs/crosschannel/commits.rst b/intl/l10n/docs/crosschannel/commits.rst
new file mode 100644
index 0000000000..955baf734f
--- /dev/null
+++ b/intl/l10n/docs/crosschannel/commits.rst
@@ -0,0 +1,33 @@
+Commits and Metadata
+====================
+
+When creating the commit for a particular revision, we need to find the
+revisions on the other branches of cross-channel to unify the created
+content with.
+
+To do so, the cross-channel algorithm keeps track of metadata associated with
+a revision in the target repository. This metadata is stored in the commit
+message:
+
+.. code-block:: bash
+
+   X-Channel-Repo: mozilla-central
+   X-Channel-Converted-Revision: af4a1de0a11cb3afbb7e50bcdd0919f56c23959a
+   X-Channel-Repo: releases/mozilla-beta
+   X-Channel-Revision: 65fb3f6bce94f8696e1571c2d48104dbdc0b31e2
+   X-Channel-Repo: releases/mozilla-release
+   X-Channel-Revision: 1c5bf69f887359645f1c3df4de0d0e3caf957e59
+   X-Channel-Repo: releases/mozilla-esr68
+   X-Channel-Revision: 4cbbc30e1ebc3254ec74dc041aff128c81220507
+
+This metadata is appended to the original commit message when committing.
+For each branch in the cross-channel configuration we have the name and
+a revision. The revision that's currently converted is explicitly highlighted
+by the ``-Converted-`` marker. On hg.mozilla.org, those revisions are also
+marked up as links, so one can navigate from the converted changeset to the
+original patch.
+
+When starting the update for an incremental graph from the previous section,
+the metadata is read from the target repository, and the data for the
+currently converted branch is updated for each commit. Each revision in
+this metadata then goes into the algorithm to create the unified content.
diff --git a/intl/l10n/docs/crosschannel/content.rst b/intl/l10n/docs/crosschannel/content.rst
new file mode 100644
index 0000000000..01f5e1ab66
--- /dev/null
+++ b/intl/l10n/docs/crosschannel/content.rst
@@ -0,0 +1,129 @@
+=====================
+Cross-channel Content
+=====================
+
+When creating the actual content, there's a number of questions to answer.
+
+#. Where to take content from?
+#. Which content to take?
+#. Where to put the content?
+#. What to put into each file?
+
+Content Sources
+---------------
+
+The content of each revision in ``gecko-strings`` corresponds to a given
+revision in each original repository. For example, we could have
+
++------------------+--------------+
+| Repository       | Revision     |
++==================+==============+
+| mozilla-central  | 4c92802939c1 |
++------------------+--------------+
+| mozilla-beta     | ace4081e8200 |
++------------------+--------------+
+| mozilla-release  | 2cf08fbb92b2 |
++------------------+--------------+
+| mozilla-esr68    | 2cf9e0c91d51 |
++------------------+--------------+
+| comm-central     | 3f3fc2c0d804 |
++------------------+--------------+
+| comm-beta        | f95a6f4408a3 |
++------------------+--------------+
+| comm-release     | dc2694f035fa |
++------------------+--------------+
+| comm-esr68       | d05d4d87d25c |
++------------------+--------------+
+
+The assumption is that there's no content that's shared between ``mozilla-*`` and
+``comm-*``, so we can just convert one repository and its branches at a time.
+
+Covered Content
+---------------
+
+Which content is included in ``gecko-strings`` is
+controlled by the project configurations of each product, on each branch.
+Currently, those are :file:`browser/locales/l10n.toml` and
+:file:`mobile/android/locales/l10n.toml` in ``mozilla-central``.
+
+Created Content Structure
+-------------------------
+
+The created content is laid out in the directory in the same structure as
+the files in ``l10n-central``. The localizable files end up like this:
+
+.. code-block::
+
+   browser/
+     browser/
+       browser.ftl
+   chrome/
+     browser.properties
+   toolkit/
+     toolkit/
+       about/aboutAbout.ftl
+
+This matches the file locations in ``mozilla-central`` with the
+:file:`locales/en-US` part dropped.
+
+The project configuration files are also converted and added to the
+created file structure. As they're commonly in the :file:`locales` folder
+which we strip, they're added to the dedicated :file:`_configs` folder.
+
+.. code-block:: bash
+
+   $ ls _configs
+   browser.toml   devtools-client.toml  devtools-shared.toml
+   mobile-android.toml  toolkit.toml
+
+
+L10n File Contents
+------------------
+
+Let's assume we have a file to localize in several revisions with different
+content.
+
+== ======= ==== =======
+ID central beta release
+== ======= ==== =======
+a  one     one  one
+b          two  two
+c  three
+d  four    old  old
+== ======= ==== =======
+
+The algorithm then creates content, taking localizable values from the left-most
+branch, where *central* overrides *beta*, and *beta* overrides *release*. This
+creates content as follows:
+
+== =======
+ID content
+== =======
+a  one
+b  two
+c  three
+d  four
+== =======
+
+If a file doesn't exist in one of the revisions, that revision is dropped
+from the content generation for this particular file.
+
+.. note::
+
+   The example of the forth string here highlights the impact that changing
+   an existing string has. We ship one translation of *four* to central,
+   beta, and release. That's only a good idea if it doesn't matter which of the
+   two versions of the English copy got translated.
+
+Project configurations
+----------------------
+
+The TOML files for project configuration are processed, but not unified
+across branches at this point.
+
+.. note::
+
+   The content of the ``-central`` branch determines what's localized
+   from ``gecko-strings``. Thus that TOML file needs to include all
+   directories across all branches for now. Removing entries requires
+   that the content is obsolete on all branches in cross-channel.
diff --git a/intl/l10n/docs/crosschannel/index.rst b/intl/l10n/docs/crosschannel/index.rst
new file mode 100644
index 0000000000..faa28d6157
--- /dev/null
+++ b/intl/l10n/docs/crosschannel/index.rst
@@ -0,0 +1,88 @@
+=============
+Cross-channel
+=============
+
+Firefox is localized with a process nick-named *cross-channel*. This document
+explains both the general idea as well as some technical details of that
+process. The gist of it is this:
+
+    We use one localization for all release channels.
+
+There's a number of upsides to that:
+
+* Localizers maintain a single source of truth. Localizers can work on Nightly,
+  while updating Beta, Developer Edition or even Release and ESR.
+* Localizers can work on strings at their timing.
+* Uplifting string changes has less of an impact on the localization toolchain,
+  and their impact can be evaluated case by case.
+
+So the problem at hand is to have one localization source
+and use that to build 5 different versions of Firefox. The goal is for that
+localization to be as complete as possible for each version. While we do
+allow for partial localizations, we don't want to enforce partial translations
+on any version.
+
+The process to tackle these follows these steps:
+
+* Create resource to localize, ``gecko-strings``.
+
+  * Review updates to that resource in *quarantine*.
+  * Expose a known good state of that resource to localizers.
+
+* The actual localization work happens in Pontoon.
+* Write localizations back to ``l10n-central``.
+* Get localizations into the builds.
+
+.. digraph:: full_tree
+
+    graph [ rankdir=LR ];
+    "m-c" -> "quarantine";
+    "m-b" -> "quarantine";
+    "m-r" -> "quarantine";
+    "c-c" -> "quarantine";
+    "c-b" -> "quarantine";
+    "c-r" -> "quarantine";
+    "quarantine" -> "gecko-strings";
+    "gecko-strings" -> "Pontoon";
+    "Pontoon" -> "l10n-central";
+    "l10n-central" -> "Nightly";
+    "l10n-central" -> "Beta";
+    "l10n-central" -> "Firefox";
+    "l10n-central" -> "Daily";
+    "l10n-central" -> "Thunderbird";
+    {
+      rank=same;
+      "quarantine";
+      "gecko-strings";
+    }
+
+.. note::
+
+   The concept behind the quarantine in the process above is to
+   protect localizers from churn on strings that have technical
+   problems. Examples like that could be missing localization notes
+   or copy that should be improved.
+
+The resource to localize is a Mercurial repository, unifying
+all strings to localize for all covered products and branches. Each revision
+of this repository holds all the strings for a particular point in time.
+
+There's three aspects that we'll want to unify here.
+
+#. Create a version history that allows the localization team
+   to learn where strings in the generated repository are coming from.
+#. Unify the content across different branches for a single app.
+#. Unify different apps, coming from different repositories.
+
+The last item is the easiest, as ``mozilla-*`` and ``comm-*`` don't share
+code or history. Thus, they're converted individually to disjunct directories
+and files in the target repository, and the Mercurial history of each is interleaved
+in the target history. When parents are needed for one repository, they're
+rebased over the commits for the other.
+
+.. toctree::
+   :maxdepth: 1
+
+   commits
+   content
+   repositories
diff --git a/intl/l10n/docs/crosschannel/repositories.rst b/intl/l10n/docs/crosschannel/repositories.rst
new file mode 100644
index 0000000000..8461b32fbd
--- /dev/null
+++ b/intl/l10n/docs/crosschannel/repositories.rst
@@ -0,0 +1,14 @@
+gecko-strings and Quarantine
+============================
+
+The actual generation is currently done via `taskcluster cron <https://treeherder.mozilla.org/jobs?repo=mozilla-central&searchStr=cross-channel>`_.
+The state that is good to use by localizers at large is published at
+https://hg.mozilla.org/l10n/gecko-strings/.
+
+The L10n team is doing a :ref:`review step <exposure-in-gecko-strings>` before publishing the strings, and while
+that is ongoing, the intermediate state is published to
+https://hg.mozilla.org/l10n/gecko-strings-quarantine/.
+
+The code is in https://hg.mozilla.org/mozilla-central/file/tip/python/l10n/mozxchannel/,
+supported as a mach subcommand in https://hg.mozilla.org/mozilla-central/file/tip/tools/compare-locales/mach_commands.py,
+as a taskcluster kind in https://hg.mozilla.org/mozilla-central/file/tip/taskcluster/ci/l10n-cross-channel, and scheduled in cron in https://hg.mozilla.org/mozilla-central/file/tip/.cron.yml.
diff --git a/intl/l10n/docs/fluent/index.rst b/intl/l10n/docs/fluent/index.rst
new file mode 100644
index 0000000000..84103db5e4
--- /dev/null
+++ b/intl/l10n/docs/fluent/index.rst
@@ -0,0 +1,25 @@
+======
+Fluent
+======
+
+`Fluent`_ is a localization system developed by Mozilla, which aims to replace
+all existing localization models currently used at Mozilla.
+
+In case of Firefox it directly supersedes DTD and StringBundle systems, providing
+a large number of features and improvements over both of them, for developers
+and localizers.
+
+.. toctree::
+   :maxdepth: 2
+
+   tutorial
+   review
+
+Other resources:
+
+ * `Fluent Syntax Guide <http://projectfluent.org/fluent/guide/>`_
+ * `Fluent Wiki <https://github.com/projectfluent/fluent/wiki>`_
+ * `Fluent.js Wiki <https://github.com/projectfluent/fluent.js/wiki>`_
+ * `Fluent DOM L10n Tutorial <https://projectfluent.org/dom-l10n-documentation/>`_
+
+.. _Fluent: http://projectfluent.org/
diff --git a/intl/l10n/docs/fluent/review.rst b/intl/l10n/docs/fluent/review.rst
new file mode 100644
index 0000000000..83d65ebed9
--- /dev/null
+++ b/intl/l10n/docs/fluent/review.rst
@@ -0,0 +1,303 @@
+.. role:: bash(code)
+   :language: bash
+
+.. role:: js(code)
+   :language: javascript
+
+===============================
+Guidelines for Fluent Reviewers
+===============================
+
+This document is intended as a guideline for developers and reviewers when
+working with FTL (Fluent) files. As such, it’s not meant to replace the
+`existing extensive documentation`__ about Fluent.
+
+__ ./tutorial.html
+
+`Herald`_ is used to set the group `fluent-reviewers`_ as blocking reviewer for
+any patch modifying FTL files committed to Phabricator. The person from this
+group performing the review will have to manually set other reviewers as
+blocking, if the original developer didn’t originally do it.
+
+
+.. hint::
+
+  In case of doubt, you should always reach out to the l10n team for
+  clarifications.
+
+
+Message Identifiers
+===================
+
+While in Fluent it’s possible to use both lowercase and uppercase characters in
+message identifiers, the naming convention in Gecko is to use lowercase and
+hyphens (*kebab-case*), avoiding CamelCase and underscores. For example,
+:js:`allow-button` should be preferred to :js:`allow_button` or
+:js:`allowButton`, unless there are technically constraints – like identifiers
+generated at run-time from external sources – that make this impractical.
+
+When importing multiple FTL files, all messages share the same scope in the
+Fluent bundle. For that reason, it’s suggested to add scope to the message
+identifier itself: using :js:`cancel` as an identifier increases the chances of
+having a conflict, :js:`save-dialog-cancel-button` would make it less likely.
+
+Message identifiers are also used as the ultimate fall back in case of run-time
+errors. Having a descriptive message ID would make such fall back more useful
+for the user.
+
+Comments
+========
+
+When a message includes placeables (variables), there should always be a
+comment explaining the format of the variable, and what kind of content it will
+be replaced with. This is the format suggested for such comments:
+
+
+.. code-block:: fluent
+
+  # This string is used on a new line below the add-on name
+  # Variables:
+  #   $name (String) - Add-on author name
+  cfr-doorhanger-extension-author = by { $name }
+
+
+By default, a comment is bound to the message immediately following it. Fluent
+supports both `file-level and group-level comments`__. Be aware that a group
+comment will apply to all messages following that comment until the end of the
+file. If that shouldn’t be the case, you’ll need to “reset” the group comment,
+by adding an empty one (:js:`##`), or moving the section of messages at the end
+of the file.
+
+__ https://projectfluent.org/fluent/guide/comments.html
+
+Comments are fundamental for localizers, since they don’t see the file as a
+whole, or changes as a fragment of a larger patch. Their work happens on a
+message at a time, and the context is only provided by comments.
+
+License headers are standalone comments, that is, a single :js:`#` as prefix,
+and the comment is followed by at least one empty line.
+
+Changes to Existing Messages
+============================
+
+You must update the message identifier if:
+
+- The meaning of the sentence has changed.
+- You’re changing the morphology of the message, by adding or removing attributes.
+
+Messages are identified in the entire localization toolchain by their ID. For
+this reason, there’s no need to change attribute names.
+
+If your changes are relevant only for English — for example, to correct a
+typographical error or to make letter case consistent — then there is generally
+no need to update the message identifier.
+
+There is a grey area between needing a new ID or not. In some cases, it will be
+necessary to look at all the existing translations to determine if a new ID
+would be beneficial. You should always reach out to the l10n team in case of
+doubt.
+
+Changing the message ID will invalidate the existing translation, the new
+message will be reported as missing in all tools, and localizers will have to
+retranslate it. This is the only reliable method to ensure that localizers
+update existing localizations, and run-time stop using obsolete translations.
+
+You must also update all instances where that message identifier is used in the
+source code, including localization comments.
+
+Non-text Elements in Messages
+=============================
+
+When a message includes non text-elements – like anchors or images – make sure
+that they have a :js:`data-l10n-name` associated to them. Additional
+attributes, like the URL for an anchor or CSS classes, should not be exposed
+for localization in the FTL file. More details can be found in `this page`__
+dedicated to DOM overlays.
+
+__ https://github.com/projectfluent/fluent.js/wiki/DOM-Overlays#text-level-elements
+
+This information is not relevant if your code is using `fluent-react`_, where
+DOM overlays `work differently`__.
+
+__ https://github.com/projectfluent/fluent.js/wiki/React-Overlays
+
+Message References
+==================
+
+Consider the following example:
+
+
+.. code-block:: fluent
+
+  newtab-search-box-search-the-web-text = Search the Web
+  newtab-search-box-search-the-web-input =
+      .placeholder = { newtab-search-box-search-the-web-text }
+      .title = { newtab-search-box-search-the-web-text }
+
+
+This might seem to reduce the work for localizers, but it actually doesn’t
+help:
+
+- A change to the referenced message (:js:`newtab-search-box-search-the-web-text`)
+  would require a new ID also for all messages referencing it.
+- Translation memory can help with matching text, not with message references.
+
+On the other hand, this approach is helpful if, for example, you want to
+reference another element of the UI in your message:
+
+
+.. code-block:: fluent
+
+  help-button = Help
+  help-explanation = Click the { help-button} to access support
+
+
+This enforces consistency and, if :js:`help-button` changes, all other messages
+will need to be updated anyway.
+
+Terms
+=====
+
+Fluent supports a specific type of message, called `term`_. Terms are similar
+to regular messages but they can only be used as references in other messages.
+They are best used to define vocabulary and glossary items which can be used
+consistently across the localization of the entire product.
+
+Terms are typically used for brand names, like :js:`Firefox` or :js:`Mozilla`:
+it allows to have them in one easily identifiable place, and raise warnings
+when a localization is not using them. It helps enforcing consistency and brand
+protection. If you simply need to reference a message from another message, you
+don’t need a term: cross references between messages are allowed, but they
+should not be abused, as already described.
+
+Variants and plurals
+====================
+
+Consider the following example:
+
+
+.. code-block:: fluent
+
+  items-selected =
+      { $num ->
+          [0] Select items.
+          [one] One item selected.
+         *[other] { $num } items selected.
+      }
+
+
+In this example, there’s no guarantee that all localizations will have this
+variant covered, since variants are private by design. The correct approach for
+the example would be to have a separate message for the :js:`0` case:
+
+
+.. code-block:: fluent
+
+  # Separate messages which serve different purposes.
+  items-select = Select items
+  # The default variant works for all values of the selector.
+  items-selected =
+      { $num ->
+          [one] One item selected.
+         *[other] { $num } items selected.
+      }
+
+
+As a rule of thumb:
+
+- Use variants only if the default variant makes sense for all possible values
+  of the selector.
+- The code shouldn’t depend on the availability of a specific variant.
+
+More examples about selector and variant abuses can be found in `this wiki`__.
+
+__ https://github.com/projectfluent/fluent/wiki/Good-Practices-for-Developers#prefer-separate-messages-over-variants-for-ui-logic
+
+In general, also avoid putting a selector in the middle of a sentence, like in
+the example below:
+
+
+.. code-block:: fluent
+
+  items-selected =
+      { $num ->
+          [one] One item.
+         *[other] { $num } items
+      } selected.
+
+
+:js:`1` should only be used in case you want to cover the literal number. If
+it’s a standard plural, you should use the :js:`one` category for singular.
+Also make sure to always pass the variable to these messages as a number, not
+as a string.
+
+Access Keys
+===========
+
+The following is a simple potential example of an access key:
+
+.. code-block:: fluent
+
+  example-menu-item =
+      .label = Menu Item
+      .accesskey = M
+
+Access keys are used in menus in order to help provide easy keyboard shortcut access. They
+are useful for both power users, and for users who have accessibility needs. It is
+helpful to first read the `Access keys`__ guide in the Windows Developer documentation,
+as it outlines the best practices for Windows applications.
+
+__ https://docs.microsoft.com/en-us/windows/uwp/design/input/access-keys
+
+There are some differences between operating systems. Linux mostly follows the same
+practices as Windows. However, macOS in general does not have good support for accesskeys,
+especially in menus.
+
+When choosing an access key, it's important that it's unique relative to the current level
+of UI. It's preferable to avoid letters with descending parts, such as :code:`g`,
+:code:`j`, :code:`p`, and :code:`q` as these will not be underlined nicely in Windows or
+Linux. Other problematic characters are ones which are narrow, such as :code:`l`,
+:code:`i` and :code:`I`. The underline may not be as visible as other letters in
+sans-serif fonts.
+
+Linter
+======
+
+:bash:`mach lint` includes a :ref:`l10n linter <L10n>`, called :bash:`moz-l10n-lint`. It
+can be run locally by developers but also runs on Treeherder: in the Build
+Status section of the diff on Phabricator, open the Treeherder Jobs link and
+look for the :js:`l1nt` job.
+
+Besides displaying errors and warnings due to syntax errors, it’s particularly
+important because it also checks for message changes without new IDs, and
+conflicts with the cross-channel repository used to ship localized versions of
+Firefox.
+
+
+.. warning::
+
+  Currently, there’s an `issue`__ preventing warnings to be displayed in
+  Phabricator. Checks can be run locally using :bash:`./mach lint -l l10n -W`.
+
+  __ https://github.com/mozilla/code-review/issues/32
+
+
+Migrating Strings From Legacy or Fluent Files
+=============================================
+
+If a patch is moving legacy strings (.properties, .DTD) to Fluent, it should
+also include a recipe to migrate existing strings to FTL messages. The same is
+applicable if a patch moves existing Fluent messages to a different file, or
+changes the morphology of existing messages without actual changes to the
+content.
+
+Documentation on how to write and test migration recipes is available in `this
+page`__.
+
+__ ./fluent_migrations.html
+
+
+.. _Herald: https://phabricator.services.mozilla.com/herald/
+.. _fluent-reviewers: https://phabricator.services.mozilla.com/tag/fluent-reviewers/
+.. _fluent-react: https://github.com/projectfluent/fluent.js/wiki/React-Bindings
+.. _term: https://projectfluent.org/fluent/guide/terms.html
diff --git a/intl/l10n/docs/fluent/tutorial.rst b/intl/l10n/docs/fluent/tutorial.rst
new file mode 100644
index 0000000000..1312efd448
--- /dev/null
+++ b/intl/l10n/docs/fluent/tutorial.rst
@@ -0,0 +1,750 @@
+.. role:: html(code)
+   :language: html
+
+.. role:: js(code)
+   :language: javascript
+
+=============================
+Fluent for Firefox Developers
+=============================
+
+
+This tutorial is intended for Firefox engineers already familiar with the previous
+localization systems offered by Gecko - `DTD`_ and  `StringBundle`_ - and assumes
+prior experience with those systems.
+
+For a more hands-on tutorial of understanding Fluent from the ground up, try
+following the `Fluent DOMLocalization Tutorial`__, which provides some background on
+how Fluent works and walks you through creating a basic web project from scratch that
+uses Fluent for localization.
+
+__ https://projectfluent.org/dom-l10n-documentation/
+
+Using Fluent in Gecko
+=====================
+
+`Fluent`_ is a modern localization system introduced into
+the Gecko platform with a focus on quality, performance, maintenance and completeness.
+
+The legacy DTD system is deprecated, and Fluent should be used where possible.
+
+Getting a Review
+----------------
+
+If you work on any patch that touches FTL files, you'll need to get a review
+from `fluent-reviewers`__. There's a Herald hook that automatically sets
+that group as a blocking reviewer.
+
+__ https://phabricator.services.mozilla.com/tag/fluent-reviewers/
+
+Guidelines for the review process are available `here`__.
+
+__ ./fluent_review.html
+
+To lighten the burden on reviewers, please take a moment to review some
+best practices before submitting your patch for review.
+
+-  `ProjectFluent Good Practices for Developers`_
+-  `Mozilla Localization Best Practices For Developers`_
+
+.. _ProjectFluent Good Practices for Developers: https://github.com/projectfluent/fluent/wiki/Good-Practices-for-Developers
+.. _Mozilla Localization Best Practices For Developers: https://mozilla-l10n.github.io/documentation/localization/dev_best_practices.html
+
+Major Benefits
+==============
+
+Fluent `ties tightly`__ into the domain of internationalization
+through `Unicode`_, `CLDR`_ and `ICU`_.
+
+__ https://github.com/projectfluent/fluent/wiki/Fluent-and-Standards
+
+More specifically, the most observable benefits for each group of consumers are
+
+
+Developers
+----------
+
+ - Support for XUL, XHTML, HTML, Web Components, React, JS, Python and Rust
+ - Strings are available in a single, unified localization context available for both DOM and runtime code
+ - Full internationalization (i18n) support: date and time formatting, number formatting, plurals, genders etc.
+ - Strong focus on `declarative API via DOM attributes`__
+ - Extensible with custom formatters, Mozilla-specific APIs etc.
+ - `Separation of concerns`__: localization details, and the added complexity of some languages, don't leak onto the source code and are no concern for developers
+ - Compound messages link a single translation unit to a single UI element
+ - `DOM Overlays`__ allow for localization of DOM fragments
+ - Simplified build system model
+ - No need for pre-processing instructions
+ - Support for pseudolocalization
+
+__ https://github.com/projectfluent/fluent/wiki/Get-Started
+__ https://github.com/projectfluent/fluent/wiki/Design-Principles
+__ https://github.com/projectfluent/fluent.js/wiki/DOM-Overlays
+
+
+Product Quality
+------------------
+
+ - A robust, multilevel, `error fallback system`__ prevents XML errors and runtime errors
+ - Simplified l10n API reduces the amount of l10n specific code and resulting bugs
+ - Runtime localization allows for dynamic language changes and updates over-the-air
+ - DOM Overlays increase localization security
+
+__ https://github.com/projectfluent/fluent/wiki/Error-Handling
+
+
+Fluent Translation List - FTL
+=============================
+
+Fluent introduces a file format designed specifically for easy readability
+and the localization features offered by the system.
+
+At first glance the format is a simple key-value store. It may look like this:
+
+.. code-block:: fluent
+
+  home-page-header = Home Page
+
+  # The label of a button opening a new tab
+  new-tab-open = Open New Tab
+
+But the FTL file format is significantly more powerful and the additional features
+quickly add up. In order to familiarize yourself with the basic features,
+consider reading through the `Fluent Syntax Guide`_ to understand
+a more complex example like:
+
+.. code-block:: fluent
+
+  ### These messages correspond to security and privacy user interface.
+  ###
+  ### Please choose simple and non-threatening language when localizing
+  ### to help user feel in control when interacting with the UI.
+
+  ## General Section
+
+  -brand-short-name = Firefox
+      .gender = masculine
+
+  pref-pane =
+      .title =
+          { PLATFORM() ->
+              [windows] Options
+             *[other] Preferences
+          }
+      .accesskey = C
+
+  # Variables:
+  #   $tabCount (Number) - number of container tabs to be closed
+  containers-disable-alert-ok-button =
+      { $tabCount ->
+          [one] Close { $tabCount } Container Tab
+         *[other] Close { $tabCount } Container Tabs
+      }
+
+  update-application-info =
+      You are using { -brand-short-name } Version: { $version }.
+      Please read the <a>privacy policy</a>.
+
+The above, of course, is a particular selection of complex strings intended to exemplify
+the new features and concepts introduced by Fluent.
+
+.. important::
+
+  While in Fluent it’s possible to use both lowercase and uppercase characters in message
+  identifiers, the naming convention in Gecko is to use lowercase and hyphens, avoiding
+  CamelCase and underscores. For example, `allow-button` should be preferred to
+  `allow_button` or `allowButton`, unless there are technically constraints – like
+  identifiers generated at run-time from external sources – that make this impractical.
+
+In order to ensure the quality of the output, a lot of checks and tooling
+is part of the build system.
+`Pontoon`_, the main localization tool used to translate Firefox, also supports
+Fluent and its features to help localizers in their work.
+
+
+.. _fluent-tutorial-social-contract:
+
+Social Contract
+===============
+
+Fluent uses the concept of a `social contract` between developer and localizers.
+This contract is established by the selection of a unique identifier, called :js:`l10n-id`,
+which carries a promise of being used in a particular place to carry a particular meaning.
+
+The use of unique identifiers is shared with legacy localization systems in
+Firefox.
+
+.. important::
+
+  An important part of the contract is that the developer commits to treat the
+  localization output as `opaque`. That means that no concatenations, replacements
+  or splitting should happen after the translation is completed to generate the
+  desired output.
+
+In return, localizers enter the social contract by promising to provide an accurate
+and clean translation of the messages that match the request.
+
+In Fluent, the developer is not to be bothered with inner logic and complexity that the
+localization will use to construct the response. Whether `declensions`__ or other
+variant selection techniques are used is up to a localizer and their particular translation.
+From the developer perspective, Fluent returns a final string to be presented to
+the user, with no l10n logic required in the running code.
+
+__ https://en.wikipedia.org/wiki/Declension
+
+
+Markup Localization
+===================
+
+To localize an element in Fluent, the developer adds a new message to
+an FTL file and then has to associate an :js:`l10n-id` with the element
+by defining a :js:`data-l10n-id` attribute:
+
+.. code-block:: html
+
+  <h1 data-l10n-id="home-page-header" />
+
+  <button data-l10n-id="pref-pane" />
+
+Fluent will take care of the rest, populating the element with the message value
+in its content and all localizable attributes if defined.
+
+The developer provides only a single message to localize the whole element,
+including the value and selected attributes.
+
+The value can be a whole fragment of DOM:
+
+.. code-block:: html
+
+  <p data-l10n-id="update-application-info" data-l10n-args='{"version": "60.0"}'>
+    <a data-l10n-name="privacy-url" href="http://www.mozilla.org/privacy" />
+  </p>
+
+.. code-block:: fluent
+
+  -brand-short-name = Firefox
+  update-application-info =
+      You are using { -brand-short-name } Version: { $version }.
+      Please read the <a data-l10n-name="privacy-url">privacy policy</a>.
+
+
+Fluent will overlay the translation onto the source fragment preserving attributes like
+:code:`class` and :code:`href` from the source and adding translations for the elements
+inside. The resulting localized content will look like this:
+
+.. code-block::
+
+  <p data-l10n-id="update-application-info" data-l10n-args='{"version": "60.0"}'">
+    You are using Firefox Version: 60.0.
+    Please read the <a href="http://www.mozilla.org/privacy">privacy policy</a>.
+  </p>
+
+
+This operation is sanitized, and Fluent takes care of selecting which elements and
+attributes can be safely provided by the localization.
+The list of allowed elements and attributes is `maintained by the W3C`__, and if
+the developer needs to allow for localization of additional attributes, they can
+allow them using :code:`data-l10n-attrs` list:
+
+.. code-block:: html
+
+  <label data-l10n-id="search-input" data-l10n-attrs="style" />
+
+The above example adds an attribute :code:`style` to be allowed on this
+particular :code:`label` element.
+
+
+External Arguments
+------------------
+
+Notice in the previous example the attribute :code:`data-l10n-args`, which is
+a JSON object storing variables exposed by the developer to the localizer.
+
+This is the main channel for the developer to provide additional variables
+to be used in the localization.
+
+Arguments are rarely needed for situations where it’s currently possible to use
+DTD, since such variables would need to be computed from the code at runtime.
+It's worth noting that, when the :code:`l10n-args` are set in
+the runtime code, they are in fact encoded as JSON and stored together with
+:code:`l10n-id` as an attribute of the element.
+
+__ https://www.w3.org/TR/2011/WD-html5-20110525/text-level-semantics.html
+
+
+Runtime Localization
+====================
+
+In almost every case the JS runtime code will operate on a particular document, either
+XUL, XHTML or HTML.
+
+If the document has its markup already localized, then Fluent exposes a new
+attribute on the :js:`document` element - :js:`document.l10n`.
+
+This property is an object of type :js:`DOMLocalization` which maintains the main
+localization context for this document and exposes it to runtime code as well.
+
+With a focus on `declarative localization`__, the primary method of localization is
+to alter the localization attributes in the DOM. Fluent provides a method to facilitate this:
+
+.. code-block:: javascript
+
+  document.l10n.setAttributes(element, "new-panel-header");
+
+This will set the :code:`data-l10n-id` on the element and translate it before the next
+animation frame.
+
+This API can be used to set both the ID and the arguments at the same time.
+
+.. code-block:: javascript
+
+  document.l10n.setAttributes(element, "containers-disable-alert-ok-button", {
+    tabCount: 5
+  });
+
+If only the arguments need to be updated, then it's possible to use the :code:`setArgs`
+method.
+
+.. code-block:: javascript
+
+  document.l10n.setArgs(element, {
+    tabCount: 5
+  });
+
+On debug builds if the Fluent arguments are not provided, then Firefox will crash. This
+is done so that these errors are caught in CI. On rare occasions it may be necessary
+to work around this crash by providing a blank string as an argument value.
+
+__ https://github.com/projectfluent/fluent/wiki/Good-Practices-for-Developers
+
+
+Non-Markup Localization
+-----------------------
+
+In rare cases, when the runtime code needs to retrieve the translation and not
+apply it onto the DOM, Fluent provides an API to retrieve it:
+
+.. code-block:: javascript
+
+  let [ msg ] = await document.l10n.formatValues([
+    {id: "remove-containers-description"}
+  ]);
+
+  alert(msg);
+
+This model is heavily discouraged and should be used only in cases where the
+DOM annotation is not possible.
+
+.. note::
+
+  This API is available as asynchronous. In case of Firefox,
+  the only non-DOM localizable calls are used where the output goes to
+  a third-party like Bluetooth, Notifications etc.
+  All those cases should already be asynchronous. If you can't avoid synchronous
+  access, you can use ``mozILocalization.formatMessagesSync`` with synchronous IO.
+
+
+Internationalization
+====================
+
+The majority of internationalization issues are implicitly handled by Fluent without
+any additional requirement. Full Unicode support, `bidirectionality`__, and
+correct number formatting work without any action required from either
+developer or localizer.
+
+__ https://github.com/projectfluent/fluent/wiki/BiDi-in-Fluent
+
+.. code-block:: javascript
+
+  document.l10n.setAttributes(element, "welcome-message", {
+    userName: "اليسع",
+    count: 5
+  });
+
+A message like this localized to American English will correctly wrap the user
+name in directionality marks, allowing the layout engine to determine how to
+display the bidirectional text.
+
+On the other hand, the same message localized to Arabic will use the Eastern Arabic
+numeral for number "5".
+
+
+Plural Rules
+------------
+
+The most common localization feature is the ability to provide different variants
+of the same string depending on plural categories. Fluent ties into the Unicode CLDR
+standard called `Plural Rules`_.
+
+In order to allow localizers to use it, all the developer has to do is to pass
+an external argument number:
+
+.. code-block:: javascript
+
+  document.l10n.setAttributes(element, "unread-warning", { unreadCount: 5 });
+
+Localizers can use the argument to build a multi variant message if their
+language requires that:
+
+.. code-block:: fluent
+
+  unread-warning =
+      { $unreadCount ->
+          [one] You have { $unreadCount } unread message
+         *[other] You have { $unreadCount } unread messages
+      }
+
+If the variant selection is performed based on a number, Fluent matches that
+number against literal numbers as well as its `plural category`__.
+
+If the given translation doesn't need pluralization for the string (for example
+Japanese often will not), the localizer can replace it with:
+
+.. code-block:: fluent
+
+  unread-warning = You have { $unreadCount } unread messages
+
+and the message will preserve the social contract.
+
+One additional feature is that the localizer can further improve the message by
+specifying variants for particular values:
+
+.. code-block:: fluent
+
+  unread-warning =
+      { $unreadCount ->
+          [0] You have no unread messages
+          [1] You have one unread message
+         *[other] You have { $unreadCount } unread messages
+      }
+
+The advantage here is that per-locale choices don't leak onto the source code
+and the developer is not affected.
+
+
+.. note::
+
+  There is an important distinction between a variant keyed on plural category
+  `one` and digit `1`. Although in English the two are synonymous, in other
+  languages category `one` may be used for other numbers.
+  For example in `Bosnian`__, category `one` is used for numbers like `1`, `21`, `31`
+  and so on, and also for fractional numbers like `0.1`.
+
+__ https://unicode.org/cldr/charts/latest/supplemental/language_plural_rules.html
+__ https://unicode.org/cldr/charts/latest/supplemental/language_plural_rules.html#bs
+
+Partially-formatted variables
+-----------------------------
+
+When it comes to formatting data, Fluent allows the developer to provide
+a set of parameters for the formatter, and the localizer can fine tune some of them.
+This technique is called `partially-formatted variables`__.
+
+For example, when formatting a date, the developer can just pass a JS :js:`Date` object,
+but its default formatting will be pretty expressive. In most cases, the developer
+may want to use some of the :js:`Intl.DateTimeFormat` options to select the default
+representation of the date in string:
+
+.. code-block:: javascript
+
+  document.l10n.formatValue("welcome-message", {
+  startDate: FluentDateTime(new Date(), {
+      year: "numeric",
+      month: "long",
+      day: "numeric"
+    })
+  });
+
+.. code-block:: fluent
+
+  welcome-message = Your session will start date: { $startDate }
+
+In most cases, that will be enough and the date would get formatted in the current
+Firefox as `February 28, 2018`.
+
+But if in some other locale the string would get too long, the localizer can fine
+tune the options as well:
+
+.. code-block:: fluent
+
+  welcome-message = Początek Twojej sesji: { DATETIME($startDate, month: "short") }
+
+This will adjust the length of the month token in the message to short and get formatted
+in Polish as `28 lut 2018`.
+
+At the moment Fluent supports two formatters that match JS Intl API counterparts:
+
+ * **NUMBER**: `Intl.NumberFormat`__
+ * **DATETIME**: `Intl.DateTimeFormat`__
+
+With time more formatters will be added. Also, this feature is not exposed
+to ``setAttributes`` at this point, as that serializes to JSON.
+
+__ https://projectfluent.org/fluent/guide/functions.html#partially-formatted-variables
+__ https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/NumberFormat
+__ https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/DateTimeFormat
+
+Registering New L10n Files
+==========================
+
+Fluent uses a wildcard statement, packaging all localization resources into
+their component's `/localization/` directory.
+
+That means that, if a new file is added to a component of Firefox already
+covered by Fluent like `browser`, it's enough to add the new file to the
+repository in a path like `browser/locales/en-US/browser/component/file.ftl`, and
+the toolchain will package it into `browser/localization/browser/component/file.ftl`.
+
+At runtime Firefox uses a special registry for all localization data. It will
+register the browser's `/localization/` directory and make all files inside it
+available to be referenced.
+
+To make the document localized using Fluent, all the developer has to do is add
+localizable resources for Fluent API to use:
+
+.. code-block:: html
+
+  <link rel="localization" href="branding/brand.ftl"/>
+  <link rel="localization" href="browser/preferences/preferences.ftl"/>
+
+The URI provided to the :html:`<link/>` element are relative paths within the localization
+system.
+
+
+Custom Localizations
+====================
+
+The above method creates a single localization context per document.
+In almost all scenarios that's sufficient.
+
+In rare edge cases where the developer needs to fetch additional resources, or
+the same resources in another language, it is possible to create additional
+Localization object manually using the `Localization` class:
+
+.. code-block:: javascript
+
+  const myL10n = new Localization([
+    "branding/brand.ftl",
+    "browser/preferences/preferences.ftl"
+  ]);
+
+
+  let [isDefaultMsg, isNotDefaultMsg] =
+    await myL10n.formatValues({id: "is-default"}, {id: "is-not-default"});
+
+
+.. admonition:: Example
+
+  An example of a use case is the Preferences UI in Firefox, which uses the
+  main context to localize the UI but also to build a search index.
+
+  It is common to build such search index both in a current language and additionally
+  in English, since a lot of documentation and online help exist only in English.
+
+  A developer may create manually a new context with the same resources as the main one,
+  but hardcode it to `en-US` and then build the search index using both contexts.
+
+
+By default, all `Localization` contexts are asynchronous. It is possible to create a synchronous
+one by passing an `sync = false` argument to the constructor, or calling the `SetIsSync(bool)` method
+on the class.
+
+
+.. code-block:: javascript
+
+  const myL10n = new Localization([
+    "branding/brand.ftl",
+    "browser/preferences/preferences.ftl"
+  ], false);
+
+
+  let [isDefaultMsg, isNotDefaultMsg] =
+    myL10n.formatValuesSync({id: "is-default"}, {id: "is-not-default"});
+
+
+Synchronous contexts should be always avoided as they require synchronous I/O. If you think your use case
+requires a synchronous localization context, please consult Gecko, Performance and L10n Drivers teams.
+
+
+Designing Localizable APIs
+==========================
+
+When designing localizable APIs, the most important rule is to resolve localization as
+late as possible. That means that instead of resolving strings somewhere deep in the
+codebase and then passing them on, or even caching, it is highly recommended to pass
+around :code:`l10n-id` or :code:`[l10n-id, l10n-args]` pairs until the top-most code
+resolves them or applies them onto the DOM element.
+
+
+Testing
+=======
+
+When writing tests that involve both I18n and L10n, the general rule is that
+result strings are opaque. That means that the developer should not assume any particular
+value and should never test against it.
+
+In case of raw i18n the :js:`resolvedOptions` method on all :js:`Intl.*` formatters
+makes it relatively easy. In case of localization, the recommended way is to test that
+the code sets the right :code:`l10n-id`/:code:`l10n-args` attributes like this:
+
+.. code-block:: javascript
+
+  testedFunction();
+
+  const l10nAttrs = document.l10n.getAttributes(element);
+
+  deepEquals(l10nAttrs, {
+    id: "my-expected-id",
+    args: {
+      unreadCount: 5
+    }
+  });
+
+If the code really has to test for particular values in the localized UI, it is
+always better to scan for a variable:
+
+.. code-block:: javascript
+
+  testedFunction();
+
+  equals(element.textContent.contains("John"));
+
+.. important::
+
+  Testing against whole values is brittle and will break when we insert Unicode
+  bidirectionality marks into the result string or adapt the output in other ways.
+
+
+Manually Testing UI with Pseudolocalization
+===========================================
+
+When working with a Fluent-backed UI, the developer gets a new tool to test their UI
+against several classes of problems.
+
+Pseudolocalization is a mechanism which transforms messages on the fly, using
+specific logic to help emulate how the UI will look once it gets localized.
+
+The three classes of potential problems that this can help with are:
+
+ - Hardcoded strings.
+
+   Turning on pseudolocalization should expose any strings that were left
+   hardcoded in the source, since they won't get transformed.
+
+
+ - UI space not adapting to longer text.
+
+   Many languages use longer strings than English. For example, German strings
+   may be 30% longer (or more). Turning on pseudolocalization is a quick way to
+   test how the layout handles such locales. Strings that don't fit the space
+   available are truncated and pseudolocalization can also help with detecting them.
+
+
+ - Bidi adaptation.
+
+   For many developers, testing the UI in right-to-left mode is hard.
+   Pseudolocalization shows how a right-to-left locale will look like.
+
+To turn on pseudolocalization, open the :doc:`Browser Toolbox <../../devtools-user/browser_toolbox/index>`,
+click the three dot menu in the top right corner, and choose one of the following:
+
+ - **Enable “accented” locale** - [Ȧȧƈƈḗḗƞŧḗḗḓ Ḗḗƞɠŀīīşħ]
+
+   This strategy replaces all Latin characters with their accented equivalents,
+   and duplicates some vowels to create roughly 30% longer strings. Strings are
+   wrapped in markers (square brackets), which help with detecting truncation.
+
+   This option sets the :js:`intl.l10n.pseudo` pref to :js:`accented`.
+
+
+ - **Enable bidi locale** - ɥsıʅƃuƎ ıpıԐ
+
+   This strategy replaces all Latin characters with their 180 degree rotated versions
+   and enforces right to left text flow using Unicode UAX#9 `Explicit Directional Embeddings`__.
+   In this mode, the UI directionality will also be set to right-to-left.
+
+   This option sets the :js:`intl.l10n.pseudo` pref to :js:`bidi`.
+
+__ https://www.unicode.org/reports/tr9/#Explicit_Directional_Embeddings
+
+Inner Structure of Fluent
+=========================
+
+The inner structure of Fluent in Gecko is out of scope of this tutorial, but
+since the class and file names may show up during debugging or profiling,
+below is a list of major components, each with a corresponding file in `/intl/l10n`
+modules in Gecko.
+
+For more hands-on experience with some of the concepts below, try
+following the `Fluent DOMLocalization Tutorial`__, which provides some
+background on how Fluent works and walks you through creating a basic
+web project from scratch that uses Fluent for localization.
+
+__ https://projectfluent.org/dom-l10n-documentation/overview.html
+
+FluentBundle
+--------------
+
+FluentBundle is the lowest level API. It's fully synchronous, contains a parser for the
+FTL file format and a resolver for the logic. It is not meant to be used by
+consumers directly.
+
+In the future we intend to offer this layer for standardization and it may become
+part of the :js:`mozIntl.*` or even :js:`Intl.*` API sets.
+
+That part of the codebase is also the first that we'll be looking to port to Rust.
+
+
+Localization
+------------
+
+Localization is a higher level API which uses :js:`FluentBundle` internally but
+provides a full layer of compound message formatting and robust error fall-backing.
+
+It is intended for use in runtime code and contains all fundamental localization
+methods.
+
+
+DOMLocalization
+---------------
+
+DOMLocalization extends :js:`Localization` with functionality to operate on HTML, XUL
+and the DOM directly including DOM Overlays and Mutation Observers.
+
+DocumentL10n
+------------
+
+DocumentL10n implements the DocumentL10n WebIDL API and allows Document to
+communicate with DOMLocalization.
+
+Events
+^^^^^^
+
+DOM translation is asynchronous (e.g., setting a `data-l10n-id` attribute won't
+immediately reflect the localized content in the DOM).
+
+We expose a :js:`Document.hasPendingL10nMutations` member that reflects whether
+there are any async operations pending. When they are finished, the
+`L10nMutationsFinished` event is fired on the document, so that chrome code can
+be certain all the async operations are done.
+
+L10nRegistry
+------------
+
+L10nRegistry is our resource management service. It
+maintains the state of resources packaged into the build and language packs,
+providing an asynchronous iterator of :js:`FluentBundle` objects for a given locale set
+and resources that the :js:`Localization` class uses.
+
+
+.. _Fluent: https://projectfluent.org/
+.. _DTD: https://developer.mozilla.org/en-US/docs/Mozilla/Tech/XUL/Tutorial/Localization
+.. _StringBundle: https://developer.mozilla.org/en-US/docs/Mozilla/Tech/XUL/Tutorial/Property_Files
+.. _Firefox Preferences: https://bugzilla.mozilla.org/show_bug.cgi?id=1415730
+.. _Unprivileged Contexts: https://bugzilla.mozilla.org/show_bug.cgi?id=1407418
+.. _System Add-ons: https://bugzilla.mozilla.org/show_bug.cgi?id=1425104
+.. _CLDR: http://cldr.unicode.org/
+.. _ICU: http://site.icu-project.org/
+.. _Unicode: https://www.unicode.org/
+.. _Fluent Syntax Guide: https://projectfluent.org/fluent/guide/
+.. _Pontoon: https://pontoon.mozilla.org/
+.. _Plural Rules: http://cldr.unicode.org/index/cldr-spec/plural-rules
diff --git a/intl/l10n/docs/glossary.rst b/intl/l10n/docs/glossary.rst
new file mode 100644
index 0000000000..f8df34e819
--- /dev/null
+++ b/intl/l10n/docs/glossary.rst
@@ -0,0 +1,22 @@
+L10N Glossary
+=============
+
+.. glossary::
+    :sorted:
+
+    Localization
+        The process of creating content in a native language, including
+        translation, but also customizations like Search.
+
+    Localizability
+        Enabling a piece of software to be localized. This is mostly
+        externalizing English strings, and writing build support to
+        pick up localized search engines etc.
+
+    L10n
+        *Numeronym* for Localization, *L*, 10 chars, *n*
+
+    l10n-merge
+        nick-name for the process of merging ``en-US`` and a particular
+        localization into one joint artifact without any missing strings, and
+        without technical errors, as far as possible.
diff --git a/intl/l10n/docs/index.rst b/intl/l10n/docs/index.rst
new file mode 100644
index 0000000000..cbd9c3e796
--- /dev/null
+++ b/intl/l10n/docs/index.rst
@@ -0,0 +1,26 @@
+============
+Localization
+============
+
+Localization – sometimes written as l10n, where 10 is the number of letters between `l` and `n` –
+is an aspect of internationalization focused on adapting software to
+different cultural and regional needs.
+
+The boundary between internationalization and localization is fuzzy. At Mozilla
+we refer to localization when we talk about adapting the user interface
+and messages, while internationalization handles operations on raw data.
+
+.. note::
+
+  Localization is a broader term than translation because it involves extensive research
+  into the target culture, and in result touches not only text and UI translation but also
+  cultural adaptation of icons, communication styles, colors, and UX.
+
+.. toctree::
+   :maxdepth: 2
+
+   overview
+   fluent/index
+   migrations/index
+   crosschannel/index
+   glossary
diff --git a/intl/l10n/docs/migrations/fluent.rst b/intl/l10n/docs/migrations/fluent.rst
new file mode 100644
index 0000000000..bc14293ed7
--- /dev/null
+++ b/intl/l10n/docs/migrations/fluent.rst
@@ -0,0 +1,153 @@
+.. role:: bash(code)
+   :language: bash
+
+.. role:: js(code)
+   :language: javascript
+
+.. role:: python(code)
+   :language: python
+
+
+===========================
+Fluent to Fluent Migrations
+===========================
+
+When migrating existing Fluent messages,
+it's possible to copy a source directly with :python:`COPY_PATTERN`,
+or to apply string replacements and other changes
+by extending the :python:`TransformPattern` visitor class.
+
+These transforms work with individual Fluent patterns,
+i.e. the body of a Fluent message or one of its attributes.
+
+Copying Fluent Patterns
+-----------------------
+
+Consider for example a patch modifying an existing message to move the original
+value to a :js:`alt` attribute.
+
+Original message:
+
+
+.. code-block:: fluent
+
+  about-logins-icon = Warning icon
+      .title = Breached website
+
+
+New message:
+
+
+.. code-block:: fluent
+
+  about-logins-breach-icon =
+      .alt = Warning icon
+      .title = Breached website
+
+
+This type of changes requires a new message identifier, which in turn causes
+existing translations to be lost. It’s possible to migrate the existing
+translated content with:
+
+
+.. code-block:: python
+
+    from fluent.migrate import COPY_PATTERN
+
+    ctx.add_transforms(
+        "browser/browser/aboutLogins.ftl",
+        "browser/browser/aboutLogins.ftl",
+        transforms_from(
+    """
+    about-logins-breach-icon =
+        .alt = {COPY_PATTERN(from_path, "about-logins-icon")}
+        .title = {COPY_PATTERN(from_path, "about-logins-icon.title")}
+    """,from_path="browser/browser/aboutLogins.ftl"),
+    )
+
+
+In this specific case, the destination and source files are the same. The dot
+notation is used to access attributes: :js:`about-logins-icon.title` matches
+the :js:`title` attribute of the message with identifier
+:js:`about-logins-icon`, while :js:`about-logins-icon` alone matches the value
+of the message.
+
+
+.. warning::
+
+  The second argument of :python:`COPY_PATTERN` and :python:`TransformPattern`
+  identifies a pattern, so using the message identifier will not
+  migrate the message as a whole, with all its attributes, only its value.
+
+Transforming Fluent Patterns
+----------------------------
+
+To apply changes to Fluent messages, you may extend the
+:python:`TransformPattern` class to create your transformation.
+This is a powerful general-purpose tool, of which :python:`COPY_PATTERN` is the
+simplest extension that applies no transformation to the source.
+
+Consider for example a patch copying an existing message to strip out its HTML
+content to use as an ARIA value.
+
+Original message:
+
+
+.. code-block:: fluent
+
+  videocontrols-label =
+      { $position }<span data-l10n-name="duration"> / { $duration }</span>
+
+
+New message:
+
+
+.. code-block:: fluent
+
+  videocontrols-scrubber =
+      .aria-valuetext = { $position } / { $duration }
+
+
+A migration may be applied to create this new message with:
+
+
+.. code-block:: python
+
+    from fluent.migrate.transforms import TransformPattern
+    import fluent.syntax.ast as FTL
+
+    class STRIP_SPAN(TransformPattern):
+        def visit_TextElement(self, node):
+            node.value = re.sub("</?span[^>]*>", "", node.value)
+            return node
+
+    def migrate(ctx):
+        path = "toolkit/toolkit/global/videocontrols.ftl"
+        ctx.add_transforms(
+            path,
+            path,
+            [
+                FTL.Message(
+                    id=FTL.Identifier("videocontrols-scrubber"),
+                    attributes=[
+                        FTL.Attribute(
+                            id=FTL.Identifier("aria-valuetext"),
+                            value=STRIP_SPAN(path, "videocontrols-label"),
+                        ),
+                    ],
+                ),
+            ],
+        )
+
+
+Note that a custom extension such as :python:`STRIP_SPAN` is not supported by
+the :python:`transforms_from` utility, so the list of transforms needs to be
+defined explicitly.
+
+Internally, :python:`TransformPattern` extends the `fluent.syntax`__
+:python:`Transformer`, which defines the :python:`FTL` AST used here.
+As a specific convenience, pattern element visitors such as
+:python:`visit_TextElement` are allowed to return a :python:`FTL.Pattern`
+to replace themselves with more than one node.
+
+__ https://projectfluent.org/python-fluent/fluent.syntax/stable/
diff --git a/intl/l10n/docs/migrations/index.rst b/intl/l10n/docs/migrations/index.rst
new file mode 100644
index 0000000000..e9ed12aa22
--- /dev/null
+++ b/intl/l10n/docs/migrations/index.rst
@@ -0,0 +1,53 @@
+.. role:: bash(code)
+   :language: bash
+
+.. role:: js(code)
+   :language: javascript
+
+.. role:: python(code)
+   :language: python
+
+=============================================
+Migrating Strings From Legacy or Fluent Files
+=============================================
+
+Firefox is a project localized in over 100 languages. As the code for existing
+features moves away from the old localization systems and starts using
+`Fluent`_, we need to ensure that we don’t lose existing translations, which
+would have the adverse effect of forcing contributors to localize hundreds of
+strings from scratch.
+
+`Fluent Migration`_ is a Python library designed to solve this specific problem:
+it allows to migrate translations from `.properties` and other legacy file formats,
+not only moving strings and transforming them as needed to adapt to the `FTL`
+syntax, but also replicating "blame" for each string in VCS.
+
+The library also includes basic support for migrating existing Fluent messages
+without interpolations (e.g. variable replacements). The typical use cases
+would be messages moving as-is to a different file, or changes to the
+morphology of existing messages (e.g move content from an attribute to the
+value of the message).
+
+.. toctree::
+  :maxdepth: 2
+
+  overview
+  legacy
+  fluent
+  testing
+  localizations
+
+How to Get Help
+===============
+
+Writing migration recipes can be challenging for non trivial cases, and it can
+require extensive l10n knowledge to avoid localizability issues.
+
+Don’t hesitate to reach out to the l10n-drivers for feedback, help to test or
+write the migration recipes:
+
+ - Francesco Lodolo (:flod)
+ - Eemeli Aro (:eemeli)
+
+.. _Fluent: http://projectfluent.org/
+.. _Fluent Migration: https://hg.mozilla.org/l10n/fluent-migration/
diff --git a/intl/l10n/docs/migrations/legacy.rst b/intl/l10n/docs/migrations/legacy.rst
new file mode 100644
index 0000000000..b1287ba4d4
--- /dev/null
+++ b/intl/l10n/docs/migrations/legacy.rst
@@ -0,0 +1,642 @@
+.. role:: bash(code)
+   :language: bash
+
+.. role:: js(code)
+   :language: javascript
+
+.. role:: python(code)
+   :language: python
+
+========================
+Migrating Legacy Formats
+========================
+
+Migrating from legacy formats (.dtd, .properties) is different from migrating
+Fluent to Fluent. When migrating legacy code paths, you'll need to adjust the
+Fluent strings for the quirks Mozilla uses in the legacy code paths. You'll
+find a number of specialized functionalities here.
+
+Legacy Migration Tools
+----------------------
+
+To assist with legacy format migrations, some scripting tools are provided:
+
+ - `XUL+DTD to Fluent`_
+ - `.properties to Fluent`_
+
+When creating a migration, one or both of these tools may provide a good
+starting point for manual work by automating at least a part of the migration,
+including recipe generation and refactoring the calling code.
+
+.. _XUL+DTD to Fluent: https://github.com/zbraniecki/convert_xul_to_fluent
+.. _.properties to Fluent: https://github.com/mozilla/properties-to-ftl
+
+Basic Migration
+---------------
+
+Let’s consider a basic example: one string needs to be migrated, without
+any further change, from a DTD file to Fluent.
+
+The legacy string is stored in :bash:`toolkit/locales/en-US/chrome/global/findbar.dtd`:
+
+
+.. code-block:: dtd
+
+  <!ENTITY next.tooltip "Find the next occurrence of the phrase">
+
+
+The new Fluent string is stored in :bash:`toolkit/locales/en-US/toolkit/main-window/findbar.ftl`:
+
+
+.. code-block:: properties
+
+  findbar-next =
+      .tooltiptext = Find the next occurrence of the phrase
+
+
+This is how the migration recipe looks:
+
+
+.. code-block:: python
+
+  # Any copyright is dedicated to the Public Domain.
+  # http://creativecommons.org/publicdomain/zero/1.0/
+
+  from __future__ import absolute_import
+  import fluent.syntax.ast as FTL
+  from fluent.migrate.helpers import transforms_from
+
+  def migrate(ctx):
+      """Bug 1411707 - Migrate the findbar XBL binding to a Custom Element, part {index}."""
+
+      ctx.add_transforms(
+          "toolkit/toolkit/main-window/findbar.ftl",
+          "toolkit/toolkit/main-window/findbar.ftl",
+          transforms_from(
+  """
+  findbar-next =
+      .tooltiptext = { COPY(from_path, "next.tooltip") }
+  """, from_path="toolkit/chrome/global/findbar.dtd"))
+
+
+The first important thing to notice is that the migration recipe needs file
+paths relative to a localization repository, losing :bash:`locales/en-US/`:
+
+ - :bash:`toolkit/locales/en-US/chrome/global/findbar.dtd` becomes
+   :bash:`toolkit/chrome/global/findbar.dtd`.
+ - :bash:`toolkit/locales/en-US/toolkit/main-window/findbar.ftl` becomes
+   :bash:`toolkit/toolkit/main-window/findbar.ftl`.
+
+The :python:`context.add_transforms` function takes 3 arguments:
+
+ - Path to the target l10n file.
+ - Path to the reference (en-US) file.
+ - An array of Transforms. Transforms are AST nodes which describe how legacy
+   translations should be migrated.
+
+.. note::
+
+   For migrations of Firefox localizations, the target and reference path
+   are the same. This isn't true for all projects that use Fluent, so both
+   arguments are required.
+
+In this case there is only one Transform that migrates the string with ID
+:js:`next.tooltip` from :bash:`toolkit/chrome/global/findbar.dtd`, and injects
+it in the FTL fragment. The :python:`COPY` Transform allows to copy the string
+from an existing file as is, while :python:`from_path` is used to avoid
+repeating the same path multiple times, making the recipe more readable. Without
+:python:`from_path`, this could be written as:
+
+
+.. code-block:: python
+
+  ctx.add_transforms(
+      "toolkit/toolkit/main-window/findbar.ftl",
+      "toolkit/toolkit/main-window/findbar.ftl",
+      transforms_from(
+  """
+  findbar-next =
+      .tooltiptext = { COPY("toolkit/chrome/global/findbar.dtd", "next.tooltip") }
+  """))
+
+
+This method of writing migration recipes allows to take the original FTL
+strings, and simply replace the value of each message with a :python:`COPY`
+Transform. :python:`transforms_from` takes care of converting the FTL syntax
+into an array of Transforms describing how the legacy translations should be
+migrated. This manner of defining migrations is only suitable to simple strings
+where a copy operation is sufficient. For more complex use-cases which require
+some additional logic in Python, it’s necessary to resort to the raw AST.
+
+
+The example above is equivalent to the following syntax, which exposes
+the underlying AST structure:
+
+
+.. code-block:: python
+
+  ctx.add_transforms(
+      "toolkit/toolkit/main-window/findbar.ftl",
+      "toolkit/toolkit/main-window/findbar.ftl",
+      [
+          FTL.Message(
+              id=FTL.Identifier("findbar-next"),
+              attributes=[
+                  FTL.Attribute(
+                      id=FTL.Identifier("tooltiptext"),
+                      value=COPY(
+                          "toolkit/chrome/global/findbar.dtd",
+                          "next.tooltip"
+                      )
+                  )
+              ]
+          )
+      ]
+  )
+
+This creates a :python:`Message`, taking the value from the legacy string
+:js:`findbar-next`. A message can have an array of attributes, each with an ID
+and a value: in this case there is only one attribute, with ID :js:`tooltiptext`
+and :js:`value` copied from the legacy string.
+
+Notice how both the ID of the message and the ID of the attribute are
+defined as an :python:`FTL.Identifier`, not simply as a string.
+
+
+.. tip::
+
+  It’s possible to concatenate arrays of Transforms defined manually, like in
+  the last example, with those coming from :python:`transforms_from`, by using
+  the :python:`+` operator. Alternatively, it’s possible to use multiple
+  :python:`add_transforms`.
+
+  The order of Transforms provided in the recipe is not relevant, the reference
+  file is used for ordering messages.
+
+
+Replacing Content in Legacy Strings
+-----------------------------------
+
+While :python:`COPY` allows to copy a legacy string as is, :python:`REPLACE`
+(from `fluent.migrate`) allows to replace content while performing the
+migration. This is necessary, for example, when migrating strings that include
+placeholders or entities that need to be replaced to adapt to Fluent syntax.
+
+Consider for example the following string:
+
+
+.. code-block:: DTD
+
+  <!ENTITY aboutSupport.featuresTitle "&brandShortName; Features">
+
+
+Which needs to be migrated to:
+
+
+.. code-block:: fluent
+
+  features-title = { -brand-short-name } Features
+
+
+The entity :js:`&brandShortName;` needs to be replaced with a term reference:
+
+
+.. code-block:: python
+
+  FTL.Message(
+      id=FTL.Identifier("features-title"),
+      value=REPLACE(
+          "toolkit/chrome/global/aboutSupport.dtd",
+          "aboutSupport.featuresTitle",
+          {
+              "&brandShortName;": TERM_REFERENCE("brand-short-name"),
+          },
+      )
+  ),
+
+
+This creates an :python:`FTL.Message`, taking the value from the legacy string
+:js:`aboutSupport.featuresTitle`, but replacing the specified text with a
+Fluent term reference.
+
+.. note::
+  :python:`REPLACE` replaces all occurrences of the specified text.
+
+
+It’s also possible to replace content with a specific text: in that case, it
+needs to be defined as a :python:`TextElement`. For example, to replace
+:js:`example.com` with HTML markup:
+
+
+.. code-block:: python
+
+  value=REPLACE(
+      "browser/chrome/browser/preferences/preferences.properties",
+      "searchResults.sorryMessageWin",
+      {
+          "example.com": FTL.TextElement('<span data-l10n-name="example"></span>')
+      }
+  )
+
+
+The situation is more complex when a migration recipe needs to replace
+:js:`printf` arguments like :js:`%S`. In fact, the format used for localized
+and source strings doesn’t need to match, and the two following strings using
+unordered and ordered argument are perfectly equivalent:
+
+
+.. code-block:: properties
+
+  btn-quit = Quit %S
+  btn-quit = Quit %1$S
+
+
+In this scenario, replacing :js:`%S` would work on the first version, but not
+on the second, and there’s no guarantee that the localized string uses the
+same format as the source string.
+
+Consider also the following string that uses :js:`%S` for two different
+variables, implicitly relying on the order in which the arguments appear:
+
+
+.. code-block:: properties
+
+  updateFullName = %S (%S)
+
+
+And the target Fluent string:
+
+
+.. code-block:: fluent
+
+  update-full-name = { $name } ({ $buildID })
+
+
+As indicated, :python:`REPLACE` would replace all occurrences of :js:`%S`, so
+only one variable could be set. The string needs to be normalized and treated
+like:
+
+
+.. code-block:: properties
+
+  updateFullName = %1$S (%2$S)
+
+
+This can be obtained by calling :python:`REPLACE` with
+:python:`normalize_printf=True`:
+
+
+.. code-block:: python
+
+  FTL.Message(
+      id=FTL.Identifier("update-full-name"),
+      value=REPLACE(
+          "toolkit/chrome/mozapps/update/updates.properties",
+          "updateFullName",
+          {
+              "%1$S": VARIABLE_REFERENCE("name"),
+              "%2$S": VARIABLE_REFERENCE("buildID"),
+          },
+          normalize_printf=True
+      )
+  )
+
+
+.. attention::
+
+  To avoid any issues :python:`normalize_printf=True` should always be used when
+  replacing :js:`printf` arguments. This is the default behaviour when working
+  with .properties files.
+
+.. note::
+
+  :python:`VARIABLE_REFERENCE`, :python:`MESSAGE_REFERENCE`, and
+  :python:`TERM_REFERENCE` are helper Transforms which can be used to save
+  keystrokes in common cases where using the raw AST is too verbose.
+
+  :python:`VARIABLE_REFERENCE` is used to create a reference to a variable, e.g.
+  :js:`{ $variable }`.
+
+  :python:`MESSAGE_REFERENCE` is used to create a reference to another message,
+  e.g. :js:`{ another-string }`.
+
+  :python:`TERM_REFERENCE` is used to create a reference to a `term`__,
+  e.g. :js:`{ -brand-short-name }`.
+
+  Both Transforms need to be imported at the beginning of the recipe, e.g.
+  :python:`from fluent.migrate.helpers import VARIABLE_REFERENCE`
+
+  __ https://projectfluent.org/fluent/guide/terms.html
+
+
+Trimming Unnecessary Whitespaces in Translations
+------------------------------------------------
+
+.. note::
+
+  This section was updated in May 2020 to reflect the change to the default
+  behavior: legacy translations are now trimmed, unless the :python:`trim`
+  parameter is set explicitly.
+
+It’s not uncommon to have strings with unnecessary leading or trailing spaces
+in legacy translations. These are not meaningful, don’t have practical results
+on the way the string is displayed in products, and are added mostly for
+formatting reasons. For example, consider this DTD string:
+
+
+.. code-block:: DTD
+
+  <!ENTITY aboutAbout.note   "This is a list of “about” pages for your convenience.<br/>
+                              Some of them might be confusing. Some are for diagnostic purposes only.<br/>
+                              And some are omitted because they require query strings.">
+
+
+By default, the :python:`COPY`, :python:`REPLACE`, and :python:`PLURALS`
+transforms will strip the leading and trailing whitespace from each line of the
+translation, as well as the empty leading and trailing lines. The above string
+will be migrated as the following Fluent message, despite copious indentation
+on the second and the third line in the original:
+
+
+.. code-block:: fluent
+
+  about-about-note =
+      This is a list of “about” pages for your convenience.<br/>
+      Some of them might be confusing. Some are for diagnostic purposes only.<br/>
+      And some are omitted because they require query strings.
+
+
+To disable the default trimming behavior, set :python:`trim:"False"` or
+:python:`trim=False`, depending on the context:
+
+
+.. code-block:: python
+
+  transforms_from(
+  """
+  about-about-note = { COPY("toolkit/chrome/global/aboutAbout.dtd", "aboutAbout.note", trim:"False") }
+  """)
+
+  FTL.Message(
+      id=FTL.Identifier("discover-description"),
+      value=REPLACE(
+          "toolkit/chrome/mozapps/extensions/extensions.dtd",
+          "discover.description2",
+          {
+              "&brandShortName;": TERM_REFERENCE("-brand-short-name")
+          },
+          trim=False
+      )
+  ),
+
+
+Concatenating Strings
+---------------------
+
+It's best practice to only expose complete phrases to localization, and to avoid
+stitching localized strings together in code. With `DTD` and `properties`,
+there were few options. So when migrating to Fluent, you'll find
+it quite common to concatenate multiple strings coming from `DTD` and
+`properties`, for example to create sentences with HTML markup. It’s possible to
+concatenate strings and text elements in a migration recipe using the
+:python:`CONCAT` Transform.
+
+Note that in case of simple migrations using :python:`transforms_from`, the
+concatenation is carried out implicitly by using the Fluent syntax interleaved
+with :python:`COPY()` transform calls to define the migration recipe.
+
+Consider the following example:
+
+
+.. code-block:: properties
+
+  # %S is replaced by a link, using searchResults.needHelpSupportLink as text
+  searchResults.needHelp = Need help? Visit %S
+
+  # %S is replaced by "Firefox"
+  searchResults.needHelpSupportLink = %S Support
+
+
+In Fluent:
+
+
+.. code-block:: fluent
+
+  search-results-need-help-support-link = Need help? Visit <a data-l10n-name="url">{ -brand-short-name } Support</a>
+
+
+This is quite a complex migration: it requires to take 2 legacy strings, and
+concatenate their values with HTML markup. Here’s how the Transform is defined:
+
+
+.. code-block:: python
+
+  FTL.Message(
+      id=FTL.Identifier("search-results-help-link"),
+      value=REPLACE(
+          "browser/chrome/browser/preferences/preferences.properties",
+          "searchResults.needHelp",
+          {
+              "%S": CONCAT(
+                  FTL.TextElement('<a data-l10n-name="url">'),
+                  REPLACE(
+                      "browser/chrome/browser/preferences/preferences.properties",
+                      "searchResults.needHelpSupportLink",
+                      {
+                          "%1$S": TERM_REFERENCE("brand-short-name"),
+                      },
+                      normalize_printf=True
+                  ),
+                  FTL.TextElement("</a>")
+              )
+          }
+      )
+  ),
+
+
+:js:`%S` in :js:`searchResults.needHelpSupportLink` is replaced by a reference
+to the term :js:`-brand-short-name`, migrating from :js:`%S Support` to :js:`{
+-brand-short-name } Support`. The result of this operation is then inserted
+between two text elements to create the anchor markup. The resulting text is
+finally  used to replace :js:`%S` in :js:`searchResults.needHelp`, and used as
+value for the FTL message.
+
+
+.. important::
+
+  When concatenating existing strings, avoid introducing changes to the original
+  text, for example adding spaces or punctuation. Each language has its own
+  rules, and this might result in poor migrated strings. In case of doubt,
+  always ask for feedback.
+
+
+When more than 1 element is passed in to concatenate, :python:`CONCAT`
+disables whitespace trimming described in the section above on all legacy
+Transforms passed into it: :python:`COPY`, :python:`REPLACE`, and
+:python:`PLURALS`, unless the :python:`trim` parameters has been set
+explicitly on them. This helps ensure that spaces around segments are not
+lost during the concatenation.
+
+When only a single element is passed into :python:`CONCAT`, however, the
+trimming behavior is not altered, and follows the rules described in the
+previous section. This is meant to make :python:`CONCAT(COPY())` equivalent
+to a bare :python:`COPY()`.
+
+
+Plural Strings
+--------------
+
+Migrating plural strings from `.properties` files usually involves two
+Transforms from :python:`fluent.migrate.transforms`: the
+:python:`REPLACE_IN_TEXT` Transform takes TextElements as input, making it
+possible to pass it as the foreach function of the :python:`PLURALS` Transform.
+
+Consider the following legacy string:
+
+
+.. code-block:: properties
+
+  # LOCALIZATION NOTE (disableContainersOkButton): Semi-colon list of plural forms.
+  # See: http://developer.mozilla.org/en/docs/Localization_and_Plurals
+  # #1 is the number of container tabs
+  disableContainersOkButton = Close #1 Container Tab;Close #1 Container Tabs
+
+
+In Fluent:
+
+
+.. code-block:: fluent
+
+  containers-disable-alert-ok-button =
+      { $tabCount ->
+          [one] Close { $tabCount } Container Tab
+         *[other] Close { $tabCount } Container Tabs
+      }
+
+
+This is how the Transform for this string is defined:
+
+
+.. code-block:: python
+
+  FTL.Message(
+      id=FTL.Identifier("containers-disable-alert-ok-button"),
+      value=PLURALS(
+          "browser/chrome/browser/preferences/preferences.properties",
+          "disableContainersOkButton",
+          VARIABLE_REFERENCE("tabCount"),
+          lambda text: REPLACE_IN_TEXT(
+              text,
+              {
+                  "#1": VARIABLE_REFERENCE("tabCount")
+              }
+          )
+      )
+  )
+
+
+The `PLURALS` Transform will take care of creating the correct number of plural
+categories for each language. Notice how `#1` is replaced for each of these
+variants with :js:`{ $tabCount }`, using :python:`REPLACE_IN_TEXT` and
+:python:`VARIABLE_REFERENCE("tabCount")`.
+
+In this case it’s not possible to use :python:`REPLACE` because it takes a file
+path and a message ID as arguments, whereas here the recipe needs to operate on
+regular text. The replacement is performed on each plural form of the original
+string, where plural forms are separated by a semicolon.
+
+Explicit Variants
+-----------------
+
+Explicitly creating variants of a string is useful for platform-dependent
+terminology, but also in cases where you want a one-vs-many split of a string.
+It’s always possible to migrate strings by manually creating the underlying AST
+structure. Consider the following complex Fluent string:
+
+
+.. code-block:: fluent
+
+  use-current-pages =
+      .label =
+          { $tabCount ->
+              [1] Use Current Page
+             *[other] Use Current Pages
+          }
+      .accesskey = C
+
+
+The migration for this string is quite complex: the :js:`label` attribute is
+created from 2 different legacy strings, and it’s not a proper plural form.
+Notice how the first string is associated to the :js:`1` case, not the :js:`one`
+category used in plural forms. For these reasons, it’s not possible to use
+:python:`PLURALS`, the Transform needs to be crafted recreating the AST.
+
+
+.. code-block:: python
+
+
+  FTL.Message(
+      id=FTL.Identifier("use-current-pages"),
+      attributes=[
+          FTL.Attribute(
+              id=FTL.Identifier("label"),
+              value=FTL.Pattern(
+                  elements=[
+                      FTL.Placeable(
+                          expression=FTL.SelectExpression(
+                              selector=VARIABLE_REFERENCE("tabCount"),
+                              variants=[
+                                  FTL.Variant(
+                                      key=FTL.NumberLiteral("1"),
+                                      default=False,
+                                      value=COPY(
+                                          "browser/chrome/browser/preferences/main.dtd",
+                                          "useCurrentPage.label",
+                                      )
+                                  ),
+                                  FTL.Variant(
+                                      key=FTL.Identifier("other"),
+                                      default=True,
+                                      value=COPY(
+                                          "browser/chrome/browser/preferences/main.dtd",
+                                          "useMultiple.label",
+                                      )
+                                  )
+                              ]
+                          )
+                      )
+                  ]
+              )
+          ),
+          FTL.Attribute(
+              id=FTL.Identifier("accesskey"),
+              value=COPY(
+                  "browser/chrome/browser/preferences/main.dtd",
+                  "useCurrentPage.accesskey",
+              )
+          ),
+      ],
+  ),
+
+
+This Transform uses several concepts already described in this document. Notable
+is the :python:`SelectExpression` inside a :python:`Placeable`, with an array
+of :python:`Variant` objects. Exactly one of those variants needs to have
+``default=True``.
+
+This example can still use :py:func:`transforms_from()``, since existing strings
+are copied without interpolation.
+
+.. code-block:: python
+
+  transforms_from(
+  """
+  use-current-pages =
+      .label =
+          { $tabCount ->
+              [1] { COPY(main_dtd, "useCurrentPage.label") }
+             *[other] { COPY(main_dtd, "useMultiple.label") }
+          }
+      .accesskey = { COPY(main_dtd, "useCurrentPage.accesskey") }
+  """, main_dtd="browser/chrome/browser/preferences/main.dtd"
+  )
diff --git a/intl/l10n/docs/migrations/localizations.rst b/intl/l10n/docs/migrations/localizations.rst
new file mode 100644
index 0000000000..0861d6e52b
--- /dev/null
+++ b/intl/l10n/docs/migrations/localizations.rst
@@ -0,0 +1,42 @@
+.. role:: bash(code)
+   :language: bash
+
+.. role:: js(code)
+   :language: javascript
+
+.. role:: python(code)
+   :language: python
+
+===========================================
+How Migrations Are Run on l10n Repositories
+===========================================
+
+Once a patch including new FTL strings and a migration recipe lands in
+mozilla-central, l10n-drivers will perform a series of actions to migrate
+strings in all 100+ localization repositories:
+
+ - New Fluent strings land in `mozilla-central`, together with a migration
+   recipe.
+ - New strings are added to `gecko-strings-quarantine`_, a unified repository
+   including strings for all shipping versions of Firefox, and used as a buffer
+   before exposing strings to localizers.
+ - Migration recipes are run against all l10n repositories, migrating strings
+   from old to new files, and storing them in VCS.
+ - New en-US strings are pushed to the official `gecko-strings`_ repository
+   used by localization tools, and exposed to all localizers.
+
+Migration recipes could be run again within a release cycle, in order to migrate
+translations for legacy strings added after the first run. They’re usually
+removed from `mozilla-central` within 2 cycles, e.g. a migration recipe created
+for Firefox 59 would be removed when Firefox 61 is available in Nightly.
+
+
+.. tip::
+
+  A script to run migrations on all l10n repositories is available in `this
+  repository`__, automating part of the steps described for manual testing, and
+  it could be adapted to local testing.
+
+  __ https://github.com/flodolo/fluent-migrations
+.. _gecko-strings-quarantine: https://hg.mozilla.org/l10n/gecko-strings-quarantine/
+.. _gecko-strings: https://hg.mozilla.org/l10n/gecko-strings
diff --git a/intl/l10n/docs/migrations/overview.rst b/intl/l10n/docs/migrations/overview.rst
new file mode 100644
index 0000000000..dc9c128fb9
--- /dev/null
+++ b/intl/l10n/docs/migrations/overview.rst
@@ -0,0 +1,136 @@
+.. role:: bash(code)
+   :language: bash
+
+.. role:: js(code)
+   :language: javascript
+
+.. role:: python(code)
+   :language: python
+
+=====================================
+Migration Recipes and Their Lifecycle
+=====================================
+
+The actual migrations are performed running Python modules called **migration
+recipes**, which contain directives on how to migrate strings, which files are
+involved, transformations to apply, etc. These recipes are stored in
+`mozilla-central`__.
+
+__ https://hg.mozilla.org/mozilla-central/file/default/python/l10n/fluent_migrations
+
+When part of Firefox’s UI is migrated to Fluent, a migration recipe should be
+attached to the same patch that adds new strings to `.ftl` files.
+
+Migration recipes can quickly become obsolete, because the referenced strings
+and files are removed from repositories as part of ongoing development.
+For these reasons, l10n-drivers periodically clean up the `fluent_migrations`
+folder in mozilla-central, keeping only recipes for 2
+shipping versions (Nightly and Beta).
+
+
+.. hint::
+
+  As a developer you don’t need to bother about updating migration recipes
+  already in `mozilla-central`: if a new patch removes a string or file that is
+  used in a migration recipe, simply ignore it, since the entire recipe will be
+  removed within a couple of cycles.
+
+
+How to Write Migration Recipes
+==============================
+
+The migration recipe’s filename should start with a reference to the associated
+bug number, and include a brief description of the bug, e.g.
+:bash:`bug_1451992_preferences_applicationManager.py` is the migration recipe
+used to migrate the Application Manager window in preferences. It’s also
+possible to look at existing recipes in `mozilla-central`__ for inspiration.
+
+__ https://hg.mozilla.org/mozilla-central/file/default/python/l10n/fluent_migrations
+
+
+General Recipe Structure
+========================
+
+A migration recipe is a Python module, implementing the :py:func:`migrate`
+function, which takes a :py:class:`MigrationContext` as input. The API provided
+by the context is
+
+.. code-block:: python
+
+  class MigrationContext:
+      def add_transforms(self, target, reference, transforms):
+          """Define transforms for target using reference as template.
+
+          `target` is a path of the destination FTL file relative to the
+          localization directory. `reference` is a path to the template FTL
+          file relative to the reference directory.
+
+          Each transform is an extended FTL node with `Transform` nodes as some
+          values.
+
+          For transforms that merely copy legacy messages or Fluent patterns,
+          using `fluent.migrate.helpers.transforms_from` is recommended.
+          """
+
+The skeleton of a migration recipe just implements the :py:func:`migrate`
+function calling into :py:func:`ctx.add_transforms`, and looks like
+
+.. code-block:: python
+
+  # coding=utf8
+
+  # Any copyright is dedicated to the Public Domain.
+  # http://creativecommons.org/publicdomain/zero/1.0/
+
+  from __future__ import absolute_import
+
+
+  def migrate(ctx):
+      """Bug 1552333 - Migrate feature to Fluent, part {index}"""
+      target = 'browser/browser/feature.ftl'
+      reference = 'browser/browser/feature.ftl'
+      ctx.add_transforms(
+          target,
+          reference,
+          [],  # Actual transforms go here.
+      )
+
+One can call into :py:func:`ctx.add_transforms` multiple times. In particular, one
+can create migrated content in multiple files as part of a single migration
+recipe by calling :py:func:`ctx.add_transforms` with different target-reference
+pairs.
+
+The *docstring* for this function will be used
+as a commit message in VCS, that’s why it’s important to make sure the bug
+reference is correct, and to keep the `part {index}` section: multiple strings
+could have multiple authors, and would be migrated in distinct commits (part 1,
+part 2, etc.).
+
+Transforms
+==========
+
+The work of the migrations is done by the transforms that are passed as
+last argument to :py:func:`ctx.add_transforms`. They're instances of either Fluent
+:py:class:`fluent.syntax.ast.Message` or :py:class:`Term`, and their content
+can depend on existing translation sources. The skeleton of a Message looks like
+
+.. code-block:: python
+
+    FTL.Message(
+        id=FTL.Identifier(
+            name="msg",
+        ),
+        value=FTL.Pattern(
+            elements=[
+                FTL.TextElement(
+                    value="A string",
+                ),
+            ],
+        ),
+    )
+
+When migrating existing legacy translations, you'll replace an
+``FTL.TextElement`` with a ``COPY(legacy_path, "old_id")``, or one of its
+variations we detail :doc:`next <legacy>`. When migrating existing Fluent
+translations, an ``FTL.Pattern`` is replaced with a
+``COPY_PATTERN(old_path, "old-id")``.
diff --git a/intl/l10n/docs/migrations/testing.rst b/intl/l10n/docs/migrations/testing.rst
new file mode 100644
index 0000000000..aa9b9747f6
--- /dev/null
+++ b/intl/l10n/docs/migrations/testing.rst
@@ -0,0 +1,58 @@
+.. role:: bash(code)
+   :language: bash
+
+.. role:: js(code)
+   :language: javascript
+
+.. role:: python(code)
+   :language: python
+
+=============================
+How to Test Migration Recipes
+=============================
+
+To test migration recipes, use the following mach command:
+
+.. code-block:: bash
+
+  ./mach fluent-migration-test python/l10n/fluent_migrations/bug_1485002_newtab.py
+
+This will analyze your migration recipe to check that the :python:`migrate`
+function exists, and interacts correctly with the migration context. Once that
+passes, it clones :bash:`gecko-strings` into :bash:`$OBJDIR/python/l10n`, creates a
+reference localization by adding your local Fluent strings to the ones in
+:bash:`gecko-strings`. It then runs the migration recipe, both as dry run and
+as actual migration. Finally it analyzes the commits, and checks if any
+migrations were actually run and the bug number in the commit message matches
+the migration name.
+
+It will also show the diff between the migrated files and the reference, ignoring
+blank lines.
+
+You can inspect the generated repository further by looking at
+
+.. code-block:: bash
+
+  ls $OBJDIR/python/l10n/bug_1485002_newtab/en-US
+
+Caveats
+-------
+
+Be aware of hard-coded English context in migration. Consider for example:
+
+
+.. code-block:: python
+
+  ctx.add_transforms(
+          "browser/browser/preferences/siteDataSettings.ftl",
+          "browser/browser/preferences/siteDataSettings.ftl",
+          transforms_from(
+  """
+  site-usage-persistent = { site-usage-pattern } (Persistent)
+  """)
+  )
+
+
+This Transform will pass a manual comparison, since the two files are identical,
+but will result in :js:`(Persistent)` being hard-coded in English for all
+languages.
diff --git a/intl/l10n/docs/overview.rst b/intl/l10n/docs/overview.rst
new file mode 100644
index 0000000000..1f1d90c105
--- /dev/null
+++ b/intl/l10n/docs/overview.rst
@@ -0,0 +1,199 @@
+.. role:: js(code)
+   :language: javascript
+
+============
+Localization
+============
+
+Localization at Mozilla
+=======================
+
+At Mozilla localizations are managed by locale communities around the world, who
+are responsible for maintaining high quality linguistic and cultural adaptation
+of Mozilla software into over 100 locales.
+
+The exact process of localization management differs from project to project, but
+in the case of Gecko applications, the localization is primarily done via a web localization
+system called `Pontoon`_ and stored in HG repositories under
+`hg.mozilla.org/l10n-central`_.
+
+Developers are expected to keep their code localizable using localization
+and internationalization systems, and also serve as localizers into the `en-US` locale
+which is used as the `source` locale.
+
+In between the developers and localizers, there's a sophisticated ecosystem of tools,
+tests, automation, validators and other checks on one hand, and management, release,
+community and quality processes facilitated by the `L10n Drivers Team`_, on the other.
+
+Content vs. UI
+==============
+
+The two main categories in localization are content localization vs UI localization.
+
+The former is usually involved when dealing with large blocks of text such as
+documentation, help articles, marketing material and legal documents.
+
+The latter is the primary type when handling user interfaces for applications such
+as Firefox.
+
+This article will focus on UI localization.
+
+Lifecycle & Workflow
+====================
+
+1) New feature
+--------------
+
+The typical life cycle of a localizable UI starts with a UX/UI or new feature need which
+should be accompanied by the UX mockups involving so called `copy` - the original
+content to be used in the new piece of UI.
+
+2) UX mockup + copy review
+--------------------------
+
+The UX mockup with copy is the first step that should be reviewed by the L10n Drivers Team.
+Their aim is to identify potential cultural and localization challenges that may arise
+later and ensure that the UI is ready for localization on a linguistic, cultural,
+and technical level.
+
+3) Patch l10n review
+--------------------
+
+Once that is completed, the next stage is for front-end engineers to create patches
+which implement the new UI. Those patches should already contain the `copy` and
+place the strings in the localization resources for the source locale (`en-US`).
+
+The developer uses the localization API by selecting a special identifier we call
+`L10n ID` and optionally a list of variables that will be passed to the translation.
+
+We call this "a social contract" which binds the l10n-id/args combination to a particular
+source translation to use in the UI.
+
+The localizer expects the developer to maintain the contract by ensuring that the
+translation will be used in the given location, and will correspond to the
+source translation. If that contract is to be changed, the developer will be expected
+to update it. More on that in part `6) String Updates`.
+
+The next review comes from either L10n Drivers, or experienced front end engineers
+familiar with the internationalization and localization systems, making sure that
+the patches properly use the right APIs and the code is ready to be landed
+into `mozilla-central`.
+
+.. _exposure-in-gecko-strings:
+
+4) Exposure in `gecko-strings`
+------------------------------
+
+Once the patch lands in `mozilla-central`, L10n Drivers will take a final look at
+the localizability of the introduced strings. In case of issues, developers might
+be asked to land a follow up, or the patch could be backed out with the help of sheriffs.
+
+Every few days, strings are exported into a repository called `gecko-strings-quarantine`,
+a unified repository that includes strings for all shipping versions of Firefox
+(nightly, beta, release). This repository is used as a buffer to avoid exposing potential
+issues to over 100 locales.
+
+As a last step, strings are pushed into `gecko-strings`, another unified repository that
+is exposed to localization tools, like Pontoon, and build automation.
+
+5) Localization
+---------------
+
+From that moment localizers will work on providing translations for the new feature
+either while the new strings are only in Nightly or after they are merged to Beta.
+The goal is to have as much of the UI ready in as many locales as early as possible,
+but the process is continuous and we're capable of releasing Firefox with incomplete
+translations falling back on a backup locale in case of a missing string.
+
+While Nightly products use the latest version of localization available in repositories,
+the L10n Drivers team is responsible for reviewing and signing off versions of each
+localization shipping in Beta and Release versions of Gecko products.
+
+6) String updates
+-----------------
+
+Later in the software life cycle some strings might need to be changed or removed.
+As a general rule, once the strings lands in `mozilla-central`, any further update
+to existing strings will need to follow these guidelines, independently from how much
+time has passed since previous changes.
+
+If it's just a string removal, all the engineer has to do is to remove it from the UI
+and from the localization resource file in `mozilla-central`.
+
+If it's an update, we currently have two "levels" of change severity:
+
+1) If the change is minor, like fixing a spelling error or case, the developer should update
+the `en-US` translation without changing the l10n-id.
+
+2) If the change is anyhow affecting the meaning or tone of the message, the developer
+is requested to update the l10n string ID.
+
+The latter is considered a change in the social contract between the developer and
+the localizer and an update to the ID is expected.
+
+In case of `Fluent`_, any changes to the structure of the message such as adding/removing
+attributes also requires an update of the ID.
+
+The new ID will be recognized by the l10n tooling as untranslated, and the old one
+as obsolete. This will give the localizers an opportunity to find and update the
+translation, while the old string will be removed from the build process.
+
+There is a gray area between the two severity levels. In case of doubt, don’t hesitate
+to request feedback of review from L10n Drivers to avoid issues once the strings land.
+
+Selecting L10n Identifier
+=========================
+
+Choosing an identifier for a localization message is tricky. It may seem similar
+to picking a variable name, but in reality, it's much closer to designing a public
+API.
+
+An l10n identifier, once defined, is then getting associated to a translated
+message in every one of 100+ locales and it becomes very costly to attempt to
+migrate that string in all locales to a different identifier.
+
+Additionally, in Fluent an identifier is used as a last resort string to be displayed in
+an error scenario when formatting the message fails, which makes selecting
+**meaningful** identifiers particularly valuable.
+
+Lastly, l10n resources get mixed and matched into localization contexts where
+it becomes important to avoid identifier collision from two strings coming
+from two different files.
+
+For all those reasons, a longer identifier such as :js:`privacy-exceptions-button-ok` is
+preferred over short identifiers like :js:`ok` or :js:`ok-button`.
+
+Localization Systems
+====================
+
+Gecko has two main localization systems: Fluent and StringBundle, a legacy system.
+
+Fluent
+------
+
+Fluent is a modern localization system designed by Mozilla to address the challenges
+and limitations of older systems.
+
+It's well suited for modern web development cycle, provides a number of localization
+features including good internationalization model and strong bidirectionality support.
+
+
+To learn more about Fluent, follow the `Fluent for Firefox Developers`_ guide.
+
+StringBundle
+------------
+
+StringBundle is a runtime API used primarily for localization of C++ code.
+The messages are stored in `.properties` files and loaded using the StringBundle API
+and then retrieved from there via imperative calls.
+
+The system provides external arguments which can be placed into the string, and
+supports basic plural categories via a proprietary API `PluralForm.sys.mjs`.
+
+Adding new StringBundle messages should only be done after serious consideration,
+and in particular any new use of PluralForm messages should be avoided.
+
+.. _Pontoon: https://pontoon.mozilla.org/
+.. _hg.mozilla.org/l10n-central: https://hg.mozilla.org/l10n-central/
+.. _L10n Drivers Team: https://wiki.mozilla.org/L10n:Mozilla_Team
+.. _Fluent For Firefox Developers: ./fluent/tutorial.html