I used to play Dota on EU servers, and every once a while a Russian joined the game (even though there are RU servers available). How could we tell?
He would always write in Russian (usually curses) and ignore all the requests to use English. It actually lead to some funny situations, where the whole team was communicating in english and he continued to spam in Russian. This was not an isolated incident and became a popular meme theme.
So, IMO, Russians prefer to write in their own language, which could explain their local section's popularity.