Why don't questions identify the native language of their author? I'm new here, but it seems to me that it might be useful (and interesting) to know that people who speak a certain language often have problems with a certain pattern in English. Maybe tags would be an appropriate measure?
Stack Exchange generally attempts to make questions accessible to a wide variety of users. Implying that a question and its answers are less valuable to part of the audience because they're native Spanish speakers —rather than the question's native-Russian-speaking asker— reduces the value of the question. The most obvious example of this is that questions must be posed in English.
The other side of this is that, similarly, answers must be in English. We don't (and shouldn't) use their native language to explain the English. A case of this is in requests for phrase or idiom explanations — if someone said What does "yolo" mean? I speak Russian, so what would the equivalent phrase be?, that's really a Russian language question. Here, we need to focus on explaining in English what "yolo" means, otherwise we'd potentially have several nearly identical questions, one for each language. This is a waste of space on the site and effort on the parts of our answerers and it also requires knowledge of another language… in the end, the answers would likely be slower to come along and lower quality.
Plus, because we're focused on English, we may not actually be able to compare their native language to English as many of us don't speak that other language. Many of our users aren't EFL/ESL teachers, multi-lingual, or linguists so we may not be able to make these comparisons. That doesn't mean our answers can't be good.
The important thing is being able to explain the concepts of the English language, not in partitioning our site into dozens of mini-sites, one for each language.