I also was wondering about why that happens, like for example when i would present a presentation in english i would speak pretty good, but when i have to present a presentation in my own native language , i just can't get the words out. I was thinking about it and figured out that the social anxiety during a presentation on foreign language would be less, because no one really expects from you to speak it perfectly, but everyone expects from you to speak fluently in you own language and therefore anxiety is even bigger. Also i knew the other students were terrible at speaking english, so that kinda calmed be down