Machine translations of user texts

[Changaco]

As machine translations have gotten better and more widely used, including to translate the Liberapay website itself, it would make sense to start using them for user-generated content as well.

Ideally Liberapay would use a Python library that provides an abstraction layer over individual translation services (DeepL, Google Translate, etc.) and could be used by other projects which fetch machine translations (for example Weblate).

Requests for machine translations should be rate-limited and might have to be accepted only from logged-in users. The retrieved translations should be cached and only refreshed after some time has passed.

Related issue: #1814.