Okay. I'm just going to start off by saying...this is the best show on television. The funniest thing I've ever seen. But it might not be appropriate.
Everything is a moderate offense, except for maybe language. Words like S**t, D**k, and A*s are used in moderation, with even an occasional F-Bomb. Since Flight of the Conchords is on HBO, there are no bleeps, so you hear the words.
The swearing, although offensive, is used in moderation. The two main characters never use swear words, instead they use substitutes. For example, Bret says "Flip" when he's mad, a song starts off with Jemaine saying "I'm the motherflippin'...", and one song is called "Mother Uckers" with the repeated lyrics "Too many mother uckers ucking with my shi".