well pak sau probably means something similar to slap block in chinese. But if I never learned the term pak sau, or forgot the term I would simply say "slap block"
Instaid of saying osotagari I have heard people say leg sweep.
Since most martial arts come from differant countries, it only makes cense that they would speak differant languages, right?
So whether you choose to pronounce the move in English, or in any other language the move is still the same.
And to answer your question we do call things by there chinese names
but I think it really doesn't make a differance.
It all depends on the instructor.
Edit: Just read your legit part,
No it doesn't make it any less "legit"
Martial Art's is mainly physical so I don't think speaking one language or speaking 9 languages makes the instructor any better or any worse