Native Finnish speaker here. Joke may be that Mika is cursing but says "kantele" instead of "perkele."

...it's a pun.
Mika's leg cramped. In Japanese it's called "足がつった"
The "Tsutta" sounds like her "Tulta"

Seika said:

...it's a pun.
Mika's leg cramped. In Japanese it's called "足がつった"
The "Tsutta" sounds like her "Tulta"

Whoa thanks i totaly missed out that i thougt it was gota orz

In hindsight, my previous comment deserves to have been downvoted. Looking at it now, I really wonder what I was thinking. That sounds like somebody threatened to shoot past-me if he doesn't explain the joke and typed the first guess that came into his mind.

Edit: "Tulta" is indeed Finnish word for (command to) "fire" and I'm just tiny bit upset about how they pronounce it in the Girls und Panzer movie but they do rather fine job considering it has "L" in it.