I do believe the mixing languages problem exists (only my own impression). Sometimes, on my end, it even causes to not have the translation available at all (as the browser thinks it’s already in my language). Maybe sometimes (often?) it still works, but I guess sometimes it doesn’t.
I have myself also had some troubles in the past where Chrome says the page can’t be translated when I try the first time. But when I try it again just a few seconds later, it then works. I didn’t pay much attention to it, I got used to the fact I often have to ask a second time for it to work. I don’t think this is related to Discourse in any way. It does it whatever the website (Discourse or not).