For anyone getting their knickers in a twist over Steve’s AOU line about “language,” just remember:
Bucky Barnes
had the world’s
biggest
potty
mouth
…And Steve was the one calling him out on it half the time.
So even if Steve Rogers does occasionally curse (and I’m not saying he wouldn’t – he grew up in Brooklyn and he was a soldier, it stands to reason he knew how), as an officer he was responsible for keeping his men reasonably respectable, and likely got into the habit of playfully chiding Bucky about his foul language.
So when Steve says “it just slipped out” – he probably forgot for a moment, in the heat of battle in winter in Europe, that he wasn’t in occupied France with the commandos, and it wasn’t Bucky cussing on the radio.
This isn’t Steve with a stick up his ass. This is Steve right out of the comics, still ridden with PTSD and occasionally forgetting which theater of war he’s in, slipping into the comfort of teasing his best friend.