The is one problem with letting Americans design things. They don't understand that the rest of the world regularly speaks more than one language, and that in different environments they're mixed up in different ways.
It's actually bliss to write Japanese.. even with my limited ability to do that. The only thing that pops up is suggestions for which kanji or whatever to use for what I entered. Works smoothly and fast.
Huh, really?
I have "Multilingual typing" enabled in my Android keyboard settings and it works pretty well. I can write a message that has English, German and French mixed in (not even that uncommon in Switzerland...) and it doesn't have any problems with it.
This is also consistent with my experience.
QuickType Keyboard: Multilingual Typing
English, Chinese, French, German, Italian, Japanese, Spanish, Portuguese, Dutch, and Hindi