The Loud siblings swearing, Leni and Lily, too. They all knew and got from their parents.
Leni did swearing, but not frequently.
Luna is the less swearing one out of her siblings.
Lily knew how to swear, those word are hard so she can't inform them out; the older siblings are trying to not swearing in front of her.
When Lincoln gets push out of the line, or his rage goes on the top, he'll swear likes he never swore. Lola is the same as him.
Lori, Luan, Lucy and Lana swearing in their thought more than by their mouth.
Lisa does swearing-whispering, that's all.
And finally, Lynn Jr, the most swearing kid in the Loud house. It's one of her hobby. Don't judge that, or she'll knock your hips off.
About the parents, when they're still dating. They were writing the letters with inappropriate words of mature adults who flirting each other. When they get married, and having children, they less swearing so the kids couldn't learned 'em. But the life never fair like a feather wouldn't never grow straight, they accidently inform those word in front of the kids, when they were learning in speaking.