Why are we allowed to call someone to the Torah by his Yiddish name? I understand that he may not have been given a Hebrew name, but would anyone even dream of calling someone up as, say, George ben Charles? Why is Yiddish treated differently than English?

I know of no reason to prohibit being called to the Torah by a non Hebrew name, but it is not common custom. I think the name given at circumsision is the one most appropriate to participating in reaffirming connection with the word of HaShem by reading from the Torah.