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Totally forgot it's a thing but you're right. They would've done a better job if they've just chosen a traditionally female pet name.
I assume Roach has connotations of it being a shortening for cockroach(male), while in polish the original name (Płotka) is not a very traditional pet name, but, it's undeniably a female pet name. Also it implies affection towards the pet since the name is "diminutive" by definition.
Blogpost polish lesson over and out.