And contrary to the iPhone you can’t even disable autocorrect! This + the super-aggressive autocorrect of watchOS (the screen is small after all so you are likely to make a mistake and we better fix it automatically!) makes it an absolute NIGHTMARE to type on an Apple Watch in multiple languages. Your only option is to use speech to type because that one for some reason works when you change the language whereas the keyboard doesn’t care.
Edit: the language switch bug on watchOS seems to have finally been fixed on watchOS 26.1. The bug was already long present on watchOS 11, so not something that watchOS 26 introduced.