Look at this:
There IS a video named "Målgruppemetoden" in this channel, a title with the Scandinavian letter "å". But if I search this title, I get nothing:
It seams this only applies for Stream, not the rest of Office 365. And yes, it also applies to the other Scandinavia letters "æ" and "ø" as well. I haven't tested, but I guess it also applies to all other non-English letters.
If I search for a sentence, let's say: "det går bra", it will find the video because of the words "det" and "bra", but if I search for "går", I'll get nothing.