I bought into the sale of MK1, because Epic Games gave a better discount, but I could not change the language. The game defaults to the system language without any options.

Solution: Right Click the game in the library -> Manage -> Launch Options and enter:

-culture=en

These are 2 letter codes like fr, it, de, es, ru, ja [Japanese] etc. The untested codes are taken from https://www.reddit.com/r/borderlands3/comments/d3g73b/how_do_i_change_the_games_language_in_the_epic/f02gm3g/