Translation tracking issue

This issue is for tracking improvements we'd like to make to our translations.

Here's a somewhat sorted list of things we want to do:

- [ ] fix the duplicate files in web/xliff
- [ ] decide on whether to keep `_` or `-` in the language tags
- Django wants `_`
- web wants `-`
- transifex?
- [ ] separate translations in the frontend between the different interfaces (flow/user/admin), to allow translators to prioritise user-facing translations
- [ ] evaluate all the translation strings we have in the backend and remove translations on the strings that are not shown in the UI (for instance, all the help_text, the verbose_name, etc.)
- [ ] ensure translation fallback is happening between languages with the same language code but different region code (for instance between `pt_PT` and `pt_BR`)
- [ ] define a clear policy (in `website/docs/developer-docs/translation.md`) about which languages we allow
- for instance, we already have `fr_FR` so we won't allow `fr_CH` or `fr_BE` since they're so similar, it would just add translation burden on our translators
- [ ] cleanup transifex teams