Eek, didn't realise that was the rules, sorry piglet, I occasionally use Cornish.
From an admin perspective, the servers are easier to monitor if you have a language that you are fluent in. I would surmise that the main reason for the rule English language only is that if someone is bullying, harassing or being mean to another player, admins can't tell if it's in a language we don't understand.
I for one am all for the primary principles of Mia like Titan, a fun friendly server for all humans
You will very rarely find me even swear (old server rules, slightly stricter than Mia)
I haven't been on for a while, but I've seen a lot of non English chat in the past year or so. If you chat in the spec team channel that should be alright as long as you keep it friendly.