Let people speak their language if they want to.
I don't know any game that tells you you have to speak a certain language
On some online communities there is enough common sense for this. I have never seen so immense usage of different language than i am seeing here.
I tried to make my czech guys learn to use english while they felt the need to chat ingame, i guess i could have just screwed that and start with our moonspeak too, we will see how you like to see my language there. From now on, i will be speaking ONLY in czech in game. And i will tell my friends to do the same, we will see if you like no english enforcing afterwards