Frankly I'm a bit worried that students who study English in an internationally minded such a Germany speak German at the university. I'd expect those students to speak English at any opportunity.
As for being better at expressing yourself on certain fields in a language not originally your own. To me that's the most normal thing in the world. I was born an raised in the Netherlands, and still live here today. But my dreams are often in English, so are all my study related thoughts, and when I'm drunk or something else

I often start talking in English. You're brain got so used to English it takes hardly any effort to switch.
Also I know this is the internet, and I know my English is far from perfect, but for an English student I think you're being a bit sloppy.