Warning: You are writing in a language that isn't the same as the user you are replying to uses! (English)

That thing keeps appearing on every post now. Sometimes it says things are Portuguese, sometimes English. I have English selected as the "Interface language", for "Content Language" I have nothing selected.

I am not sure what is broken to cause that to pop up all the time, but you can remove it with a bit of custom css added to your profile:

.reply_form_inner > .alert {display: none;}  

I am on mobile and can't really verify in the codebase that this won't also hide any other potential flashed alerts that might show up at this position. I tried to be quite specific with the css selector to avoid a false positive match.

It's supposed to inform you about a mismatch in languages. Either the person you're replying to set the language incorrectly. Or you did. There's a "More options" underneath the text area to set the language of a reply.

I'd try to set the language metadata correcty... Otherwise posts and replies might be hidden from some people. For people who didn't set anything for "Content Language", every post will show up. For someone who for example selected English and French in their settings, all the posts set to Portuguese will not be displayed, doesn't matter if someone misconfigured the post, or if it's really Portuguese.

I'll try this, thanks!