Lot's of people are complaining about the people not speaking in English on Euro and US servers and trolling other all the way. Blizzard needs to keep attention on this point as their did on other games.
You cannot stop a person from joining a English server if he doesn't want to. MOBA games need to be a team play and that issue is hard to solve, unless they come with a real time language transcriber/translator for in-game text chat. That tool is clearly a possible thing to do.[1]
Since thats a third party tool it isn't perfect but platform developers could work on something similar and smart.
[1] Dota2 Translator http://sletmo.com/ / https://github.com/patriksletmo/Dota2Translator
So there is no easy solution for this problem.