Why can't it be 'the girls are in the window'. If you are look a window and there is a bunch of girls, wouldn't it be most fitting to say 'the girls are in the window?' I think this translation should be excepted.
I was pondering that myself. "In the window" does imply you are seeing them from outside, and can have other connotations depending on the neighborhood (ahem), so perhaps Italian has a similar distinction.
Yes, you always use the masculine plural if you are referring to a plural noun which can be either multiple male countable nouns, or male and female countable nouns. If you are referring to e.g. a group of only women, then you use the fem. Plural.