Seems my Android only can have one language at a time.
Discussion
Strange, which android version are you using?
Android 11 on a Xiaomi.
Super strange, it says here that it should be possible: https://www.techbone.net/xiaomi/user-manual/add-keyboard-language
Yes I already have English and Japanese there.
Ok, so you do have two languages, but they don't show up when you click in Auto on a translated message?
No just the currently set system language. I temporary changed the system language to English which let me change the target translation language to English as a workaround.
Then changed the system language back to Japanese.