If someone were learning English, but didn’t know enough to ask in it, could they ask a question in their native tongue here? On Spanish, French, one of the Russian sites, etc. this is allowed, so how about here?
The short answer is “no”. There is more detail in this post on meta: Do posts have to be in English on Stack Exchange
If someone asks a question in a language other than English, they severely limit the people able to answer their question, and the number other learners who can benefit from it.
The community is happy to help learners improve their posts. We only ask that people write to the best of their ability, not that they be perfect.
If a question is asked in a language entirely other than English, that's grounds to vote to close it as "unclear what you're asking."
If it's obvious that English is not the person's native language, then allowances are made for mistakes in the English being used. Either it can be edited to correct obvious mistakes or the gist of the question can (hopefully) be understood well enough to have an answer provided.
Phrases in a foreign language are fine. Especially if the question is something like this:
In [some language] there is the saying [some foreign phrase]. It means [English description]. Is there an English equivalent?