Something I never thought about until now with the High and Low Speech

I've read the series several times and for the first time I had a thought about the High and Low speech and wondered if I'm right. During book 4 Wizard and Glass someone says something in the High Speech and I just had a thought that it's not even english. For years I just thought maybe it was a fancy way of talking. But now I think that it might be a whole different language like Italian. What do people think? Is the High Speech fancy english or something completely different?