i18n: adds a check on missing translation keys in locale/*.json file

This commit adds a script to check all keys from translation files to
make sure no missing keys / no translation files are incorrect.

Even if missing keys should not happen (because we use a script to add
keys automatically) this script made helped to understand that all
three `ar`, `eo` and `fr_FR` translation files were completly off
compared to the `en.json` file.

I believe we can safely delete `ar`, `eo` and `fr_FR` locales from
Zanata (because those languages are not set in the ALLOWED_LANGUAGES
config array). ⚠️ this needs to be check in the production `config.php`
file ⚠️.

Deleting the `fr_FR` locale file should solve both !358 and #395

closes !358 #395