Bucky is actually a natural polyglot. Languages come easily to him. Of course, there was Romanian, Russian, Italian… basically, Bucky probably has a good conversational grasp of languages spoken by the various immigrant communities in Brooklyn. That includes the swearing.
But nobody in the army is surprised at the salty language. Bucky is a sergeant to boot, with a knack for getting the guys in line. And of course, the minute he starts talking, it’s pure Brooklyn coming out of his mouth. He can get in very fine form once provoked.
Now, nobody expects the same thing out of Steve Rogers. Super serum or no, he’s still got that sweet, innocent look to him. The phrase “butter wouldn’t melt in his mouth” was used, sending Bucky into fits of laughter. He knew his best friend, for all his natural good nature, had that Irish temper and no, it was never a good idea to rouse that side of Steve Rogers.
The first time Steve completely lost his patience and started swearing was due to a fellow captain who was known around the entire regiment for what everyone called chickenshit stunts. Everyone was stunned. After a moment, Dum Dum was heard to remark that “Cap somehow managed to turn swearing into a whole new language. Beautiful. Thought only Barnes could do it.”
Bucky wasn’t the only one with the knack for language after all.