I have a friend who's freakin' hilarious when she swears in Creole. I don't need to understand the language to know what she means. 
I'm not bilingual but I speak some vietnamese and sometimes swear in vietnamese but mostly english. as it's my native language
There's a higher possibility of me swearing in English than Polish. I just kinda stuck with "no swearing" policy when I'm speaking my first language. I might swear if I'm furious or something, but usually I'll avoid it. I drop f-boms in English sometimes mostly as a joke.
Also for some reason English swears seem "mild" to me compared to most languages. xD
I swearing in my mother tongue, Spanish, is more natural to me. Here in my country is socially acceptable to curse (a lot🤣). When I am speaking English I swear in English but to me the words sound "softer" than Spanish bad words: "f***k" is not as offensive as "ch*** a tu m***" . So if I am very angry I am more likely to curse in Spanish.
Mostly in english. There's just something relatively clean about saying fuck compared to my mother tongue's equivalents - maybe because I hear so few of them in non-vulgar/aggressive contexts. I still swear a little bit in portuguese but like, a mild word or two, never anything harsh.
When i swear, i do in the language i am speaking at the moment.
But when i stub my toe or something like that, i will swear in spanish.
Yeah, the Spanish language has a poetic way of swearing that hits further on people`s souls XD
My native language is Bruneian Malay and English is my second language, and few other basic east asian language. But I tend to pick up swear words from other languages... so the language I use when swearing is quite colourful? But I mostly use Chinese and Korean so that my parents don't understand it
I swear almost always in Spanish. I have way more room for creativity there hahaha. From time to time I'll mutter to myself f*** or something like that when something isn't working, but nothing else. Oh wait, yes, I also use wtf irl hahaha I've gotten that habit and people around me use it as well so... It works.
Also I still have trouble wrapping my head around the fact that swearing isn't perceived the same way in Spanish (at least in Spain) than in English. Gotta get used to using hell instead or f*** for now svdkgk
I can speak 3 language (sort of...?), and I mostly swear with the Chinese dialect one, although in online and me swearing out loud (it's interchangeably with the Chinese dialect tho), it's usually in English. Very very rarely searing in Indonesian although it's my most proficient language I have.
I swear in both Danish, English, and Korean 
Mostly use Korean because no one else understands it around here.
I swear in Dutch, i can swear in English but Dutch gives them a little extra oomph because it's my mother's tongue.
English and German.
My dad was really good at not swearing in his language when me and my siblings were kids so I never picked up Lao/Thai swears until I was older, so the 'it's just a habit I've had since a kid' doesn't click in for swears in that language.
Unlike German... where my mom and her side of the family thought 'if we swear in German the children will not know what we're talking about and then will not learn them' NOPE me, my siblings and all my cousins knew how to swear in German before we knew how to swear in English. To the point that for most of us... it's really the ONLY German words we know x.x
I swear in Russian, it's my native language and I automatically switch to it when emotions take over.
My father is french and my Mom German, so i grow up learning the two langage at the same time.
Before, I often swear in German. But since i live with my husband and speak more often French than german, i swear also more often in French. I think it depends on the environment you are in.
Also, outside, when i'm surrounded by French people i will swear in german, and when i'm surrounded bu Germnans i swear in French. I guess it's a bit due to respect and i try to not chock people to much (You never know if a child will hear you)...
In England the stock phrase for apologising when you're about to deliberately swear is "Pardon my French", but the words that are being apologised for sound rather more like Anglo Saxon to me.
haven’t had the chance to read all the comments but yeah, most people who are bilingual to the point it’s like, very natural speech in both languages, will swear in their not-mother-tongue. not sure why rly!
i do it myself though with time i got used to swearing in my mother tongue too. still more natural in english tho
I swear mostly in my first language, which is Dutch when I am really really really mad. When I am slightly mad it is usually English.lol
I toggle 2 languages all the time in my day to day life, because my beloved only speaks English.
If I'm having a conversation in English, I swear in English.
If I'm having a conversation in my native language, I swear in that.
Whichever language I last used (be it in conversation, text, or song) tends to be the language that stays "on". So even when I'm alone in my thoughts, the language that is "on" is the one I think and swear in.
Oh man, I hope what I wrote makes sense...
Hi, technically bilingual here - I speak English and Tagalog/Filipino. I usually swear in English, because although Tagalog is my native language, I'm not actually very comfortable speaking it neither am I VERY GOOD at it because I grew up as an english speaker (living abroad and all that). My brain is kind of just automatically hardwired to English, haha.
Though sometime out of instinct or if I'm already engaged in a conversation with someone in Tagalog, I probably will say putang ina, lmao.