Serafina understood exactly what Kazia was referring to, and her scowl became sour. This was a topic they've talked about plenty of times in the past, but it wasn't one Serafina enjoyed talking about. She didn't like foreigners as it was, let alone having to work with them in something as serious as this. 

There was also another reason, one a bit more serious. 

Serafina's voice became a tad more serious, hoping her friend would pay attention to her words. "Kazia, understand that not everybody is willing to be nice and cooperate. You might be used to working as a team, but many prefer to fight on their own. They'll have no qualms leaving you to die, and would never want to share. Just because we're friends doesn't mean everybody will be. "

The life of a magical girl was usually that of being alone. You have much more to gain working on your own, and most would probably see it that way. Kazia might be used to working with magical girls, but she was probably just lucky in the past. Luck like that wasn't likely to strike twice, and Serafina had to make sure her friend was aware of that.

She'd also only work with fellow Poles, but Kazia probably knew that already.

Probably.