Re: European language
"there is only space for one motherlanguage in your brain. At least, that was what I always heared. Is there a new theory?"
There is.
"English would probably be best, although Esperanto was created as a language pretty much everyone in the world should be able to learn."
Esperanto is heavily based on indo-european languages (it uses their gramatic structures etc.). Other language groups (like austro-asian) will have it a lot harder to learn it.
"Culture is very important. The importance of ones mother tongue is harder to understand in large nations because their language has never bee under threat of going extinct. There are just 1 mil Estonian speakers in the world . My country was under foreign rule from 1210-1918. 700!! years of being occupied by Sweden,Denmark,Poland,Russia, the germans and templar knights."
I wouldn't really call it "occupied" (at least, not most of them). I'm making a study round the subject this year at uni (although, it's about the burgundian netherlands), and without going into detail: There's a significant difference between being occupied and having an external monarch.