feat: Add i18n

[Kout95]

What

• It allows to have translations in frontend.

To translate we use lit-localize, which is a library that uses lit-element to handle translations. The translations are stored in the translations folder in the root of the project.

To add a new translation you need to :

• add msg in your code like this https://lit.dev/docs/localization/overview/#message-types
• run npm run extract:translations to extract the new translations
• add your translation with 'target' tag in the xliff/<your_language>.xlf files
• run npm run build:translations to update the translations in the src/generated/locales/<your_language>.js file

I add command :

• npm run translations build before npm run build to be sure translations is generated in frontend/localization/generated folder
• add pre-commit check to verify every translation files in frontend/xliff is up to date with node command npm run translations:extract

For the moment I only add "fr", and "en" is the default language

For crowdin, I identify this for .xlf files https://store.crowdin.com/xliff

Part of

• https://github.com/orgs/openfoodfacts/projects/96/views/2?pane=issue&itemId=67157055