So I figured out how to write part of a chapter in another language (e.g. Sindarin) and get past the pop-up thingy were it says "This story does not appear to be in [insert language], please select the correct language."

You write the part in, lets just say Sindarin, and then you go change your story's "official language" or whatever to "other". Then you can publish the chapter and change the story's language back to whatever it was before.

I think this works.
Fingers crossed.