Since nobody believe me, I made a video (sorry about bad quality, too tired to play with mobile):
https://www.youtube.com/watch?v=VjkPfLAg3rQ
This is what I've done:
1. I edit one ePub, for 10-15 minutes. Everything is fine, changing between language works fine (both ways: alt+shift and alt+shift+1...)
2. At one point, after changing language (with ALT+SHIFT+2), Sigil crashes
3. After that, every time I start Sigil, even without any ePub open, and I press ALT+SHIFT+2, it crashes. It does not crash when I change language with ALT+SHIFT