The idea makes sense in light of this being a language forum.
Here are the reservations though.
- A few people might be unsure what to put. I.e. people who moved to another country.
- It's not that unusual to be fully bilingual.
- Just because a person's native tongue is Russian doesn't mean he or she isn't able to provide very good assistance with English questions, or vice versa. Perhaps better than native speakers in some cases.
- It could get "political" or "ethnic " or whatever you want to call it. I.e. a few people might for example feel compelled to put "Ukrainian" even though they are just as good at Russian - just as an example. Or "Welsh" to make a statement, even though odds are they are much better at English in that case. This might cause confusion among people who don't know the background.
- Also we have all our "secret" friends from the political forum. They like to pretend they know everything about every area and are completely objective.If they had to state their native language their bias would be revealed and their little mystery game would be up.