I'm Russian, though I moved to the US at age 5 so English is my 'native' language, even though it's technically not.

I see this on both sides. I find American curse words harsher than Russian ones, but my parents are the opposite. I can drop a "fuck" and no one really minds, drop a "suka" (roughly equates to "bitch" in Russian), my parents will loose their minds.

I’m sure my parents would do the same if I dropped one in English, even though they are not native English speakers.

Googling for 'swearing in second language' gives lots of articles about how it feels less intense. Eg

https://www.connexionfrance.com/Mag/Language/Swearing-in-a-s...

https://www.theguardian.com/science/blog/2017/mar/27/bad-lan...

Same thing here. I think the difference is whether the swear words are ingrained as part of "absorbing" the language as a child, as opposed to learning it "properly" as an adult. Basically, whether you only understand their meaning rationally, or it's a deeply ingrained taboo.