Allow for machine translation from other locales

[eemeli]

This is an offshoot from considering adding LLM features for improving machinery translations (#3068, #3069).

Consider adding similar functionality that uses other languages' translations as the source. In LLM terms, the query could be something like "Given that X is translated to $other_language as Y, how would it be translated to $this_language?" This could also be achieved with an NMT translation from the other language.