It is mandatory for Google Play.

Metadata directories were renamed while adding store descriptions into
strings files for simplicity.

This interferes with releases so is released.
An hardcoded lookup table is used instead.

* CI: Run 'check_layout.py'

Ensures that 'check_layout.output' is not outdated.
Also, update it.

* CI: Check new store descriptions

When a store description is added, 'python3 sync_translations.py' leaves
an untracked file that is not checked.

This makes sure that untracked store descriptions are noticed in CI.

This makes translation easier as there's a single file to edit at.
Existing short and full descriptions are conserved.

sync_translations.py takes care of updating the metadata files.

The metadata directories are renamed to match the language codes used in `res/`.

Contributing guidelines are updated accordingly.