English.
Profanities don't have the same energy in Welsh.
Sgwt anifeiliaid.
Yes, it's kind of the same in Spanish. Chatting with some Catalans (people from Barcelona), they told me that they swear in Spanish because Catalan has no swear words. Hard to believe but it seems that swear words appear only in official languages.
This is what I like the most about Europe, the richness of its culture. You travel to a nearby city and people are speaking in another language, even in the same country. and the customs change a bit. In Latinamerica is different, it's boring. No matter where you go people speak Spanish everywhere and have the same fkn customs. Maybe in Brasil things change but Portuguese is very similar to Spanish. It only changes the pronunciation of the words but in the end it's the same fkn thing. And if you think that going up to the US you can see new stuff, you'll find out that it's all the same. In Florida, Texas and California, people speak Spanish, no need of English. You have to go deeper, further north, to find people that speak English and have different customs.