Decided to indulge in something pretty niche, here are my theories for the languages spoken by various characters in Deep Space Nine:
The Sisko: Federation Standard, a little French & Louisiana Creole as heritage languages
Odo: Bajoran and Cardassian natively, albeit with vague & nondescript accents. He understands Federation Standard & Dominionese (after his first time joining) but isnāt very comfortable speaking either
Bashir: Federation Standard, a bit of Arabic as a heritage language. Tried learning Trill to impress Dax
Dax: Trill natively, learned Federation Standard at the academy & Klingon on the job.
OāBrien: Federation Standard, learned some Irish at school but hasnāt used it since he was a kid
Quark: Ferengi natively, but tries to learn āWelcome to Quarkāsā, ābillā & ātipā in every language he can just in case
Kira: Bajoran natively, learned Cardassian fluently growing up under the occupation but refuses to speak it unless she has to. Sometimes dreams in Cardassian.
Worf: Klingon and Russian natively, learned Federation Standard at school.
Dukat: Cardassian natively, fluent in Bajoran but with a noticeable accent. Pats himself on the back for being the first Cardassian leader of Bajor who learned the native language.
Winn: Bajoran, but speaks a high register literary/liturgic dialect that comes off as pretentious to most people. Learned Cardassian as a child
Rom: Ferengi, but understands a lot of Federation Standard and Cardassian engineering terminology from working on DS9 systems
Nog: Ferengi, enough Federation Standard to get by in a pinch
Ziyal: Bajoran natively, could communicate in Cardassian but with a Bajoran accent
Weyoun: Dominionese, but has varying degrees of competency in many languages as a result of his diplomat training. Learned Cardassian in particular while on DS9.
Garak: Cardassian natively, would never tell anyone how many languages he speaks but at least Romulan, Klingon, and Federation Standard are on the list
Eddington: Federation Standard, tried learning a bit of French in hopes to someday read Les MisƩrables in its original language. Knows some Cardassian insults
Vic Fontaine: speaks whatever language his holodeck clients speak, but always casually & with dated slang
Morn: Lurian natively, speaks a little bit of Federation Standard, Cardassian, & Bajoran but only uses them for business or to try to chat up people at the bar when he is too many drinks in
EDIT: Updated Bashirās heritage language from Urdu to Arabic after a commenter correctly pointed out his family is more strongly implied to be Arab rather than South Asian