This is what we know:
- on the North
- cold (students wear warm clothes)
- Grindelwald attended it (he was German if I'm not mistaken)
- Igor Karkaroff is a Slavic sounding name
- Krum was Bulgarian
- Poliakoff is the name of one Durmstrang student
- they came with a boat
- Krum swims in some lake there
- the name is German sounding

Bulgaria
Pro: Krum is Bulgarian, students have Slavic names, Bulgaria is close to countries which were part of German-speaking Austro-Hungary
Contra: not on the north, warm, the name doesn't sound Bulgarian, students wouldn't need warm colthes

Germany
pro: the name sounds German, Grindelwald attended it and he was German, plenty of lakes
contra: it's not no the north

Russia
pro: cold (think of the clothes Durmstrang students wear), students have Slavic sounding names (also Igor Karkaroff, the headmaster), Krum says the school is on the north
contra: the name of the school isn't Slavic

Iceland
pro: cold (clothes, again) Krum says the school is on the north, cold weather (clothes), Durmstrang students came with a boat
contra: nothing Slavic

Switzerland
pro: German speaking country (so everything that goes for Germany works here too), cold, plenty of lakes for Krum to swim in
contra: not on the north

Norway
pro: on the north, cold, lakes
contra: Slavic ties? (it's close to Russia, however)

I'm thinking Norway

Bulgaria!!!!!

Wow, you've really thought this out, I'm impressed.

I always figured it was Russia or the Czech Republic / Slovakia area, but now that I'm looking at a map and your excellent points, I'm thinking CR & S may be too far south. But then again, what do I know, I've never been there, I don't know how the weather is. =P Guess I'll stick with Russia.

it's here in Estonia. no kidding

the Dumbledore-Gridlewald big battle takes place on 1945 and there is a lot of WW2 references in there with Numengard and all...i'll put my bet on Germany

Norway maybe

I definitely think it was Russia. You see, the cold, the Slavic names, the fact that Krum was Bulgarian (same alphabet, similar language), north, their uniform. Plus, people tend to compare Voldemort and Co. with the Nazi Germany. If that's correct, you'd probably bet on another big enemy, Russia to be referred to. They also said Durmstrang was famous for Dark Magic teaching. And I don't see any reason for the others you mention.
I don't think it's Germany. I just never imagined it that way. Germany isn't that North. Definitely not Bulgaria, because that SOUTH from Great Britain.