Is there a law against saying obscenities? If yes, what is considered obscene? Does it apply to any language or only to Hebrew?
The Talmud (Shabbos 33a) states (using Is. 9:16 as a prooftext) that obscene speech causes various national troubles, G-d forbid. It then goes on to say:
"Rabbi Chanan bar Rava says: Everyone knows why a bride enters the bridal chamber. Nevertheless, if one speaks obscenely about it, even if there was a Divine decree that he enjoy seventy years [the average human lifespan] of goodness, it can thereby be turned into bad."
So there may not be a formal black-letter law against obscene speech, but we see from this passage that it is definitely something to be avoided. To the best of my knowledge, there is no difference in this regard what language is used. As for "what is considered obscene," perhaps we might best adopt Justice Potter Stewart's definition: "I know it when I see it."
As a punishment for obscenity, troubles multiply, cruel decrees are proclaimed afresh, the youth of Israel's enemies. (Shabbat 33a)
All know for what purpose a bride enters the bridal canopy, yet against whomsoever who speaks obscenely [thereof], even if a sentence of seventy years' happiness had been sealed for him, it is reversed for evil.(Shabbat 33a)
Even a superfluous conversation between a man and his own wife is mentioned to him at the time of his death. (Chagiga 5b)
Therefore the Lord shall have no joy in their young men, neither shall He have compassion on their fatherless and widows; for every one is ungodly and an evil-doer, and every mouth speaketh wantonness. For all this His anger is not turned away, but His hand is stretched out still. (Isaias 9:16)
(from messilat yesharim)