I'll give the big ole, overly specific answer, "it depends."
If it's directed at themselves, I will almost never sanction. If I hear it, but it's not that loud, I usually ignore it. If I'm pretty sure others hear it, I might just say to the player, "Language..." which usually tells them instantly what I mean, and it's not a problem.
If it's directed at me or my partner specifically, sanction.