On the one hand I think they should sing in Polish just to keep the interest in their music in Poland.But on the other hand
Their call, unless they massacre the English language as some overseas pop groups do.
There was that German one, Sarah something, who sang in English with a British /Irish name. Her song was nevertheless too kitschy and German to do well in most markets. It lacked authenticity and was puzzling why she wouldn't use her own German name.
en.m.wikipedia.org/wiki/Sarah_Connor_(singer)