asking and explaining in English is difficult for them
Exactly, that's why they should practice English. Immersion is very important when acquiring a language. On English Language Learners, we see almost exclusively questions by users whose first language is not English. As long as they try to be clear, our regulars can figure out what they mean.
For example, it would be extremely useful to describe differences between synonyms or synonymic phrases.
It regularly happens that a question is asked like "In my native language, we have [word x] and [word y] which English seems to translate with a single [word z]. Can I really use it in this situation (where we always use [word x]) and in that situation (where we always use [word y])." One often does not need to be familiar with that language to explain how English handles it.
English SE site for Russian language questions exists.
Yes. (Though it's actually bilingual; some questions are actually asked in Russian.) I guess Stack Exchange makes an exception for English, since English is the standard language of the network. All staff members speak English, so they can intervene if things get out of hand on an English site; AFAIK there's only one who speaks Russian.