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.
  - leftOffset (Double): If set, the left offset of
    this key, relative to the key immediately to the
    left, or the start of the row. If not set, the key
    will be implicitly located at the right of the
    previous key in the row. Multiples of 0.5 are
    accepted.
  - width (Double): Relative width of the key in the
    row. 1 is for a square key. Multiples of 0.5 are
    accepted. The implicit default value is 1.
  - height (Double): Relative height of the key in
    the grid. The key will overflow downwards. Multiples
    of 0.5 are accepted. The implicit default value is 1.
  - 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
