"Wy" (plural you) don't work for single person in Polish, it sounds like "communist-speach" to us (almost like you called people "tovarishch") :) It was only used by soviet puppet politicians during communism (as carbon copy of Russian expression).
So it needs "Ty" (singular you), and with singular you "sure" translates to "pewien" for male recipients and "pewna" for female.
In Polish it should be "Jesteś pewny(or pewien)/pewna, że chcesz wyjść?"
So, it's Polish-specific, not Slavic-specific as I thought.
How is that typically handled in software? I can also imagine non-software examples where this might be hard (A sign reading "Warning: you are entering a restricted area", etc.)
I explained poorly, verbs in second person are gender agnostic, adjectives are gender-dependent.
The "restriced area" is funny, standard version is "Nieuprawnionym wstęp wzbroniony" ~ "For non-priviledged-ones entry is forbidden", it's plural noun made from adjective, in nominative it would be different for male and mixed/female groups (uprawnieni/uprawnione), but fortunately in plural in dative case it's "uprawnionym" for both genders so it works out OK. I guess it's common and maybe that's why the dative case works that way?
In most dialogs in software adjectives are the problem, especially "are you sure". Usually software that don't know your gender for other reasons just use male version.
In non-software world in formal documents it's often written like "he/she" in every gender-dependent place, often with passive voice to cut the amount of "/".
So it needs "Ty" (singular you), and with singular you "sure" translates to "pewien" for male recipients and "pewna" for female.
In Polish it should be "Jesteś pewny(or pewien)/pewna, że chcesz wyjść?"
So, it's Polish-specific, not Slavic-specific as I thought.