I think a little language theory is enough to hand wave a lot of these complaints.
Bahamut is "Bahamut" in more than one culture because he is a real, proven to exist, deity. He has told people his name and continues to do so. There isn't necessarily a murky past where his name was translated from one language to another several times over without any definitive source to clarify the pronunciation.
If we assume common = english (which is a little arrogant, but is the connection most American gamers make in my experience) then we could also assume that Bahamut's name is not in common. It's a name from some other, possible dead, root language that has merely been integrated into common. One again the presence of a proven, and active, deity saying "this is my name, respect me and get it right if you want my aid" is more than enough to keep it from becoming garbled by translation.
All imho of course. I've run games where gods were known by different names in different cultures, but the gods were significantly less active in day-to-day life than the bar set by the typical D&D setting.
