Language of Durmstrang
It's supposed to be in northern Scandinavia (according to a post summing up a reading she did in 2000), but the name seems based on a German phrase and I'm thinking (based on name, location and Krum being Bulgarian) it sort of encompasses Germanic central Europe, Scandinavia and eastern Europe. Based on the name, I'm tempted to say German, but...I don't know. I really don't.
The name Durmstrang seems to be an anagram of the well-known German phrase Sturm und Drang, which means "Storm and Stress." I had always pictured it as being in Germany, but drawing students from the entire northern and eastern Europe area. It seems reasonable that a school would have a name derived from the language in which the school is located.
The number of wizarding schools in Europe is unclear. Book 4 mentions the Triwizard Tournament as having been established 700 years ago among the three larger schools in Europe, so we know that at around 1400 there were also some smaller schools, but whether they still survive, and where they draw their students from, is unknown. It is hard to believe that northern Scandinavia was sufficiently developed in the 1300-1400s to have established a school of wizardry.
Hm, yeah, I was iffy about a magical school in Scandinavia, but that's where she's said it is...? And up there with all the snow (and furs) makes sense with what we know, if not necessarily with history. I wonder if maybe Germanic people established it? The Muggles had an empire going through a large portion of the middle ages according to my atlas (though not in Scandinavia. Hmmph).
This is why I try not to think too hard about some things (and fail utterly). I was just going to go on the assumption Durmstrang is there Because Jo Said So and just wonder what language is spoken.
I think it depends on how closely you want to link especially modern Muggle with wizarding history. A lot of people in the north-east speak Russian, German not so much. The older generation sometimes do, because of WWII occupation or because it was German territory up to some point in the 20th century, but Russian had a wider reach, especially since some other languages resemble it more... The school is definitely not in Germany, because German weather is actually kinder than English, and no one runs around in furs or bathes in ice lakes...
Sweden became an important player in the 17th century, so not in the relevant time period, but it's not so hard to imagine, say, vikings establishing a fort somewhere which later would become Durmstrang... Scotland wasn't necessarily a huge player in that time either, yet that's where Hogwarts is. With magical transportation as an option, it wouldn't really matter where you put your school. Just based on climate and a sort of gut feeling, I'd place the school somewhere between Finland and Estonia.
The question of language is not just where the school is located, but also which language is the native language of most of its students and teachers...