I apologize then. Thank you guys for informing me of my ignorance.
So question then what is your Greek name and do you guys normally go by the American translation of your name when you live in America, or Greek origin when visiting America?
Realize this question was basically already answered. Thus, some of what I've written below is redundant.
No need to apologize. The English translation of his name is somewhat confounding to me and others.
I believe Americans pronounce his name Gee arn us. Typically, it's pronounced Yia knes (with a long e sound). And, is spelled Yianis or Yiannis, not Giannis.
More commonly you don't hear the "s" at the end of the name. Rather, it's said, 'Yiani'.
Since I don't read Greek news sources, I'm not sure if the 's' is left out in newsprint or not, or simply not pronounced. My feeling is it's written, not always pronounced. (I could be wrong here. if anyone knows please correct me.)
I know in America, Greeks more commonly say "Yiani", not "Yianis".
To answer your question.
The name Yianis is commonly translated into the name John in English. Most first and second generation Greeks use John rather than Yiani(s). However, it's not uncommon for first generation Greeks to be called Yiani when conversing within the Greek community. Some second or third generation Greeks use Yianis, but I believe it's relatively rare.
Native born Greeks often go by Yianis instead of John. Thus, we have Giannis Antetokounmpo.