Extremely odd glitch - Switched from Russian to German in middle of an exercise!
So I'm strengthening my Russian in untimed practice, when all of a suddenly this appears.
As you can see from the flag by my username, I'm definitely in Russian, so this was a little startling (I literally did a double take).
The rest of the strengthening exercise was in German from this point onwards.
This could have been a consequence of the extension scripts I use to make Duolingo a bit easier to use, I suppose, but if not, definitely seems like something you should know about.
Did you have another tab in german opened in the background? This has happened to me before with various languages, I had say, a Swedish lesson opened in one tab, and I was currently working on an Irish lesson in another tab, and all of the sudden I had a Swedish translation! Duo doesn't work well when different lessons are open in different tabs.
Hope this helped!