I think that, with decent machine translation widely available, basic “is this spam” or “is this hostile” moderation can be done even by moderators who don’t speak the language. So, I’m thinking we might as well allow them — what’s the harm?
If a new language becomes really popular and there are long discussions, we will probably be able to recruit a new moderator to handle that.
I also don’t see a real problem with posting both in English and a native language — sure, it’s a double-post, but in a different category with a different audience.
The only thing I’d like is for someone to help me with tag naming for these languages. When we discussed this before folks strongly favored tags for languages using that language. This is fine, but as a monolingual English speaker, I don’t know the nuances of parts of speech of other languages. In this case, I guess русский — but I’m not 100% sure that’s right.
I think that there is no need to restrict the languages. Either allow only English or allow any language. There are translation tools available and dedicated moderators who speak the native language are not a definitive need.
I also agree with your point that if a language becomes popular, we will have people who can recruit to help. To be honest, I don’t think that will happen. The reality is that frequent posters will probably realize the English language topics get more attention and non-English language topics will not overwhelm the forum and can be translated manually.