Most people assume it's impossible to swear in Celestial, but that's not entirely true. While it is true that most most profanities in most languages don't translate well (if at all) to Celestial, that doesn't mean the language has its own take on swearing; swearing in Celestial is more like giving a very long, eloquent, and almost formal "Reason You Suck" speech to the recipient- detailing in no uncertain terms exactly what is wrong with them and/or condemning them (proportionately) for it without actually making any use of what would be considered profanity in other languages (though what it does refer to would still probably give pause to anyone who could understand it).

The "art" of cursing in Celestial is pretty much only restricted to native-speaking Outsiders, though; the fact that each word translates in nearly every other language to about a page's worth of very specific complaints against a particular kind of problem in detail, combined with a fact that Celestial has a nearly unquantified number of these very context-sensitive "curses", makes it nigh-impossible for non-native speakers to learn to properly use them. Some still attempt to do so anyway, but this tends to just draw weird looks from those who know the language better.