Well, not quite muggle, but totally mud-blood. Like, it was this pure Germanic language, and Low German was this ultimate language of fantasy and adventure. Also, Danish, because of Hans Christian Anderson. Awesome, magical and whatnot. And then weird stuff happened. Invasions of the Normans, French-speaking upper-class, et cetera. Now it's bizarrely confusing and complicated. There are even more complicated languages out there, and I've done some research on them, but it still sucks that the language is irregular at all. It's basically like a highly irregular Esperanto, an ugly mish-mash of other languages.

So ironically, the language used by Slytherins is very impure. If Slytherins valued their purity, they would all speak Frisian or Low Saxon. Or, you know, go the romantic rout and speak straight-up Latin, because wouldn't that be pretty useful for learning and creating their own spells? Either way, given the worldview of Slytherins, they would consider English a language for Muggles.