Now for a real challenge, what are some swear words in Catalan? >:)
The most important one: fotre (or cardar, in the Girona area), which means "to fuck". But it's used differently than in English. In English you add "fucking" between words (for example: "the fucking best"), but in Catalan it substitutes the word. For example:
Fot-li més sal: literally "fuck more salt", meaning "pour more salt"
Quin fred que fot: literally "it fucks so cold", meaning "it is so cold"
It can also be used as the adjective "fotut" (m) / "fotuda" (f) meaning bad. For example, if you just found out you have an exam tomorrow and you haven't studied, you might say "estic fotut" or "ho tinc fotut" ("I'm fucked"). But "estar fotut" is also often used to mean that you're ill.
"Fotut" or "refotut" ("re-fucked", for emphasis) can also sometimes be used like the English "fucking": "no em parlis d'aquell fotut examen!" ("don't even mention that fucking exam!"). Nowadays, many people use the word "puto"/"puta" ("whore") in this way, as the result of influence from Spanish.
But the most common in this situation is "cony" ("cunt"): "què cony fas?" ("what the fuck are you doing?"), "he deixat aquell cony de feina!" ("I've quit that fucking job!").
A lighter version of this is "dimonis" ("devils"), though this is so light it's not really considered a swear word and it's quite outdated: "què dimonis fas?" ("what the hell are you doing?")
Another one we say all the time is "collons!". It's a vulgar word for testicles, but it's used as an interjection that can have all sorts of meanings depending on your intonation: suprise, admiration, anger, exasperation, incredulity...
Besides that, adding this word to other sentences is also very common to express a full range of meanings and emotions. I've done my best to add subtitles to a video that talks about it, but it was difÃcil de collons (difficult as fuck). I've translated it as "balls", thought it's a stronger word in Catalan, more of a curse word.
(As you can see, some of those have a sexist implication)
Though probably what we say the most is "me cago en..." ("I shit on...") as the equivalent of the English "fuck!" as a quick reaction when you drop something or get hurt, something bad happens, or you want to complain or insult. Some common ones are:
Me cago'n Déu: I shit on God
Cago'n dena or cago'n deuna: I shit on G... (euphemism to not say "god", like when English speakers say "heck" to avoid saying "hell")
Me cago'n tot: I shit on everything
Me cago'n tu: I shit on you
Me cago'n la mare que et va parir: I shit on the mother that gave birth to you
Me cago'n la mare que et va matricular: I shit on the mother that registered you (to avoid the word "give birth", which can be a swear word by itself)
Me cago'n la mar salada: I shit on the salty sea
Me cago'n Judas: I shit on Judas
Me cago'n Satanàs (quan era jove): I shit on Satan (when he was young)
Me cago'n l'hòstia: I shit on the communion bread
Me cago'n l'hòstia consagrada: I shit on the blessed communion bread
Me cago'n la puta or me cago'n la grandÃssima puta: I shit on the whore or I shit on the greatest whore
Cago'n cony: I shit on the cunt
These are only a few examples of the most common ones. There are many more, and these ones can get longer. For example, "me cague en un barril d'hòsties amb el cap de Déu per tapadora" ("I shit on a barrel of communion bread with God's head as the lid") or "me cago'n els claus-ganxo que aguanten les postetes del cagadero de Cristo" ("I shit on the nail-hooks that hold up Christ's shitting hole"), among many others. There are many fixed expressions like those, but people are also welcome to let their imagination come up with new ones related to the context and the people it's directed at.
And the soft, family friendly equivalent of those is "caso'm l'olla" or "caso'm l'os pedrer".
We also very often say "merda!" ("shit!") and "hòstia!" (this word is both a vulgar way of saying getting slapped or crashing into something and the communion waffle, it's used like English's "damn it!", "shit!" or "fuck!").
And to say something is great, we say "de puta mare!" ("of a whore mother!").
Of course, besides those ones there's the insults and ways to tell people to go to hell. But we'll leave those for another day (believe me, that will be a long post).
It's also worth saying that we use swear words less carefully than English speakers generally do. Many parents will say "fotre" in front of their kids or on TV and it's not that much of a big deal.