Well if it's appropriate to the character I will. Running my current Planescape game there's a lot of 'Pike off' 'Pike it' 'Piking' 'Sodding', etc. Though this led to problems when I was also playing in a Shadowrun campaign. I swear, I almost got my slangs confused between those games at times. Somehow the Harmonium officer calling you a bunch of 'orc drek' doesn't make sense in Sigil...
But in that Shadowrun campaign I was playing a Zory v Zakona in the small russian mob in Seattle in that game. While I didn't curse, in english, alot, my players learned some of more functional bits of russian that I've been able to retain. Most of that language I've lost, having once been fluent in it, since North Carolina has very few people you can speak in it to...