A quote from Harry Potter and the Goblet of Fire, chapter 16: The Goblet of Fire:
But this query was answered almost instantly; they were level with the Slytherin table now, and Karkaroff had just bustled up to his students.
“Back to the ship, then,” he was saying. “Viktor, how are you feeling? Did you eat enough? Should I send for some mulled wine from the kitchens?”
Harry saw Krum shake his head as he pulled his furs back on.
“Professor, I vood like some vine,” said one of the other Durmstrang boys hopefully.
“I wasn’t offering it to you, Poliakoff,” snapped Karkaroff, his warmly paternal air vanishing in an instant. “I notice you have dribbled food all down the front of your robes again, disgusting boy —
Harry can obviously understand them so they surely speak English. Poliakoff even has an accent.
But why do they? It makes little sense that they use English when talking to each other.