In no particular order: English, Esperanto, and a smattering of Quebec French (which is freakishly different than just about anywhere else).
English, except for one Italian oath I picked up from my high school orchestra teacher.
Anglish
German
Spanglish
Cuban Spanish
Canuck French
Cree
It could be a coincidence, or may be I’ve been swearing in Cockney dialect all this time, I’m cool with both possibilities 
In fact, I prefer the Cockney origin, always felt bad swearing in Orcish, that’s why I did it very ocassionally.
German (mother language)
English
Finnish
French
In that order of frequency. Although, there is no order of severity, I guess (maybe French for the weaker stuff).
Danish, English, Swedish and Finnish in that order. I only know one swearword in Finnish, but PERKELE can sound very cathartic. Remember to roll the R ;).
I forgot PERKELE!, I use it ocassionally after seeing it mentioned in the Scandinavia and the World webcomic.
Initially I pronounced it wrong (with emphasis in the “KE”, as is natural in spanish, but then I saw a video of a finnish man chasing a bear from his backyard and learned to pronounce it correctly)
My wife swears at me in Mandarin, so I’ve picked up some of that. Generally I stick to English, though.
I think y’all’ve just proven that orcs speak Cockney.
It’s certainly true that the trolls in The Hobbit speak Cockney. According to John Rateliff, this is because that dialect was easily recognizable by the intended audience of the story, Tolkien’s children. The Orcs in LOTR also tend toward Cockney (more so than the goblins in The Hobbit). Tolkien’s rationale would probably be that Cockney represents the degraded, low-class manner in which Orcs spoke the Common Speech.
Fucking English.
English, Portuguese, German, French, Spanish and Dog.
German and English, mixed. You would hear me scream “Verdammte Scheiße! Fuck! Shit! Kacke!” after I fucked up something.
Orkish: pushdughâsh! (filthy shitfire!)
<tangent>How potent is “Verdammte”? I ask because it appears to translate as “Damned” or “Damn it” and those are fairly light as English curse words go, well below the F word, but not uttered in church. For that matter, is “Scheiße” equal in strength with “Shit”, which is a notch above “Damn” on the mom-will-wash-your-mouth-out-with-soap scale?</tangent>
“Verdammt” itself is very mild, even church going ladies would utter “Verdammter Mist”, with “Mist”, meaning “dung”, far more milder than “Scheiße”. I’ve often heard the theory that German swear words are rather prone to relate to excretions than English ones that tend to be rather about sexual things, because of different social taboos. I don’t have any clue if that is a sound theory.
Same here.
If things are not so bad and I’m in a place where swearing is discouraged I might utter what I understand to be a Swedish old lady swear. I like saying it and no one can really object.
I’ve also been known to shout a very bad Polish word when the mood strikes me.
“By me” – mostly, English – I being English, and a lifelong UK inhabitant. Sometimes favoured, a few choice bits from other languages: French / Russian / German / Hungarian. I particularly like the German – faced with exasperating / maddening circumstances – “Ach du meine Scheisse !” (Oh you my shit !) – wondrously illogical, but highly expressive.
I gather (chiefly from Harry Turtledove’s “Southern Victory / TL 191” alternative-history series, which has a segment involving Quebecois characters); that on the basis of “swearing is about ‘talking dirt on’ what you most highly esteem” – with Quebec having been in times past anyway, one of the most solidly Catholic places in the world: its bad language revolves around the names of the various kinds of gear used in the Catholic Mass. As you say – it’s different.
Lovely one, that – come across by me, in writings by a couple of people involving Finland / Finnish characters. The word means, I gather, in Finnish, “devils”.
I seem to recall Tolkien’s mentioning in one of his commentaries on LOTR, that his rendering of Orcs’ conversation was – ugly though it was in published form – nonetheless considerably cleaned-up: their regular everyday exchanges would have been of the kind – familiar in certain human milieux – where every other word was an obscenity. Tolkien, being a very “proper” kind of guy, and a devout Catholic, wouldn’t have wanted to get deep into that.
Yes, that was in Appendix F to LOTR: