Add support for querying all languages at once with TranslationQueryset

This PR adds support for passing `'all'` to `TranslationQueryset.language()`.
It has many uses, such as finding an object by translated field when the language the value is in is not known (for instance, when working with translatable slugs).

Especially, now that `language()` is lazily resolved, implementation is trivial. Most of the PR is testcases.