Concrete information on Japanese input for your list:

For English input, being able to submit comments/chats with the enter key is a feature, but for Japanese (and maybe Chinese?) users it's a bug.

In Japanese the enter key is used multiple times to select from multiple possible characters while composing a sentence—it's just how Japanese input works. If an app submits when enter is pressed, it's very difficult to finish typing even a single sentence. Often the only solution is to compose text elsewhere and then copy/paste.

There is a solution: check the keyboard event isComposing (https://developer.mozilla.org/en-US/docs/Web/API/KeyboardEve...) property (alternatively, oncompositionstart and oncompositionend) and only submit if false.