* 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Ensures that 'check_layout.output' is not outdated.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.