This song was played by Mika.
Mika (ミカ) from Girls und Panzer showed up as supporting character in Girls und Panzer der FilmGirls und Panzer der Film. SheShe is the commander of the Continuation High School BT-42 tank and she likes to play Säkkijärven polkka with her kantele whenever she has the chance to do so. And oh boy how wonderful her tank moved when she was playing it in the movie. ❤