Each site can ask to disable the Enter from submitting comments.
For example Chinese Language SE asked it here: Disable [Enter] in comment field and it was indeed done, two years ago.
This is the reason given there:
My IME allows me to choose from several hanzi and commits the selected hanzi by typing [Enter]. At the moment I have to edit the comment for every hanzi I enter (which also means I can write a comment for at most 5 minutes).
Which does make sense. So I'd say that any language-specific site has a high chance of disabling Enter from submitting comments for that reason.
More sites where Enter is disabled from submitting: (Feel free to add more)