Honestly, I've always felt pretty "m'eh" about all that. Like, I do get that those words are offensive slang terms, but not in the country of origin of the figure. I'm not even sure if those are still popular terms over in England, or if they've become fairly old and removed from the popular culture. Someone more familiar with British English could let me know.
I more or less grew up in a metalworking shop, so Slag has never meant anything else to me but the burnt metal residue that I had to sweep off of the floor. As such, it's a great name for a fire-breathing Dinobot. When a word has multiple meanings, and the user of the word so clearly and obviously is using it in the inoffensive, actual-definition-of-the-word sense, it feels like you almost have to be trying to be offended because the word also has a more obscure but offensive slang connotation in another country.
That said, though, it doesn't bother me at all when Hasbro puts "Slug" on the package. I don't generally keep packaging anyway, so it could say Rock Strongo or Princess Ponyweather III for all I care. He'll be Slag to me.