You can no more, in general, say “What is preposition p in language l” than you can say “What is the Latin for the German dative?” But with closely related languages you can identify cognates, or at least words doing cognate jobs. And bottles of soap or shampoo are often miniature bathroom Rosetta stones.
cu, it turns out, is Romanian for “with”. This looks, alongside Spanish con and Italian con, to be a descendant of Latin cum. But there are two languages with plenty of speakers that don’t use a cognate for that job.
There’s no commonly-used single-word French translation for “both”, which is odd because Latin had ambo. This is the same as the Essex placename Wendens Ambo. Catalan uses it to mean “with”, in the names of dishes and concerts.
Do we all know about avec? Unusually for a French preposition it can be stranded at the end of a sentence. But where does it come from? According to my French dictionary it’s from Old French avuec which is itself from apud hoc, or “at the house of that”. But what I don’t have readily to hand is whatever happened to cum in French and Catalan.