My Grandmother died when I was a baby. My mother said I called her Nani. My Grandfather who is still around to this day amazingly. I call him Papa.
Paqua and great paqua (grandfather and great grandfather) I could never say grandpa as a kid and every time I tried it came out like that. Now I say grandfather or say grandpops, depends who I'm talking to.
Well, when I refer to the grandparents on my dad's side, I stick by the mandarin pronounciation even if I'm speaking in english. It's pronounced "yie yie" for grandpa, and "nai nai" for grandma. For the ones on my mom's side, I say "gong gong" for grandpa and "poh poh" for grandma, since they're cantonese.