Add possibility to limit available languages

Example case: a trainer would like international participants to complete the training in either English or Finnish. The aim is to also improve language skills, so they'd want participants to not do it in e.g. Spanish.

Currently the option is to either have translations on for all languages or not at all, and to produce the same content in multiple languages, which would cause more admin work and separate participants. It would be nice if admins could specify languages the content can be translated to, disabling the rest.