When we look at the real world, diplomats or even heads of state will sometimes swear at each other...some recent examples are Hugo Chavez telling Álvaro Uribe to go to hell in the recent Latin America and Carribean heads of state meeting in Mexico, or that UK Independent MEP launching a public tirade against Herman Van Rompuy. Iran and North Korea make sometimes vulgar comments about the US, and back in the day, Canadian and US leaders have also publically called each other sons of bitches before. Even if there are norms out there which stipulate swearing is not something you do in the diplomatic sphere, it happens.
As orathaic pointed out, or at least alluded too, swearing could be a game changer for some people in some instances. If I swear, even at no one in particular, you could be offended and attack me. As doofman has also pointed out, swearing could be used as a tactic to rile someone up, particularly if they don't like it. Diplomacy is a game about personalities; swearing is or isn't part of someone's personality, and thus it effects the game accordingly depending on the other personalities playing.
As there is nothing about swearing in the rules, you could create a variant where there is no swearing allowed, so I disagree with the option of a foulmouth game because it isn't really a variant at all (although one could advertise it in the title of the game). However, judging by the maturity level of some of the players on this site, I can see some people abusing a 'clean talk' game, giving the moderators another layer of problems to deal with. And as some other members have mentioned, there is a rather subjective level of what constitutes swearing or dirty language.
So, in short, swearing to me is part of the game, but by all means, go ahead and make a variant if you want, but proceed at your own risk.