I was trying to reply in Chinese to a private message also written in Chinese, and the notification that kept popping up said, “Body seems unclear, is it a complete sentence?”
I have not encountered this issue before, though I wonder if it’s because of the proper Chinese punctuation used (full-width punctuation instead of half-width punctuation used in English), and how there is and is supposed to be no space between words and punctuation.
Weird, I copied the text and it can be sent as a reply, but not as a new message.
I suspect it’s more of Discourse disallowing all caps in a post body, triggered by the phrase UCPC (Note that non-Latin characters also played a role here as Discourse essentially overlooked their presence).
Deleting the phrase UCPC or inserting a lowercase character in the message body may fix it.
Alternatively @paulo could consider ticking “Allow uppercase posts: allow all caps in a topic title or a post body.” checkbox in admin settings. (Ultimately Discourse needs a better text processing to accommodate non-Latin characters)
Thank you so much 
! Using lower case letters for UCPC worked and I am able to send the post.
I’m going to edit out the content of my message from this thread since the issue is solved.
I kept getting the same error message (that leads to nowhere) when attempting to reply to a Japanese thread, where people were discussing in Japanese. The user I was replying to had a upper case in her name, which I removed, and I was able to send the reply… but it’s very inconvenient 
I hope this will get fixed somehow…?
I have already submitted a suggestion for a fix to Discourse, but they haven’t act on it. If I find the time I’ll try to submit a specific implementation that I believe would address it.