Gnome-shell OSK layouts are extracted from CLDR layout definitions:
https://www.unicode.org/cldr/charts/latest/keyboards/layouts/index.html

To add new layouts from CLDR:

1) Run update-osk-layouts.sh

2) Modify JSON files to add extra keys, tweak appearance...

2) Do git add and git commit

JSON file format
================

Each JSON file describes a keymap for a certain language and layout,
it has the following structure:

- Root (Object)

  Root object of a layout, has the following properties:

  - levels (Array of Level): Levels in the keymap
  - locale (String): Locale name for this keymap
  - name (String): Human readable name for this keymap

- Level (Object)

  A level defines the keys available on a keyboard level,
  these are the key panels visible at a time. E.g. to type
  uppercase levels or symbols.

  Levels have the following properties:

  - level (String): Name of the level, common names are
    "", "shift", "opt" and "opt+shift".
  - mode (String): Mode for this level, common modes are
    "default", "latched" and "locked".
  - rows (Array of Row): Array of rows of keys.

- Row (Array)

  A row is an Array of Key.

- Key (Object)

  A keyboard key. Keys have the following properties:

  - iconName (String): Icon name to show on the key.
    Keys with an icon name do not have a label.
  - label (String): Label to show on the key.
  - strings (Array of String): Strings to commit. If
    label property does not exist, the first element
    will be also the label. Extra elements are shown
    in the extra keys popover.
  - keyval (String): Hexadecimal keyval to emit as
    emulated key presses. Committed strings are
    preferred.
  - width (Double): Relative width of the key in the
    row. 1 is for a square key. Multiples of 0.5 are
    accepted.
  - level (Integer): Level that the key switches to.
    See the levelSwitch action.
  - action (string): Action performed by the key,
    accepted actions are:

    - hide: Hides the OSK
    - languageMenu: Pops up the language selection
      menu
    - emoji: Switches to the emoji selection panel
    - modifier: Handles the keyval as a modifier
      key. This handles e.g. Ctrl+A as a sequence
      of Ctrl press, A press, A release, Ctrl
      release.
    - delete: Deletes text backwards
    - levelSwitch: Switches OSK to a different level