I speak English very well. I have spoken it in professional environments for so long that, in a regular conversation, even accent barely reveals that I'm a non-native speaker anymore. However, my own native language is part of my cultural heritage, a heritage that I deeply appreciate, which is very dear to me and which cannot be adequately rendered in English any more than The Canterbury Tales can be adequately rendered in my own language.
We stick to our local languages because they're part of who we are. Europe would probably have a little more money if we stopped doing that, but it would be a lot poorer.