Why was the Osiris Egyptian God Card translated as Slifer the Sky Dragon?
The name Osiris was changed to Slifer by 4Kids employee Sam Murakami, possibly due to religious symbol/reference.

According to Wikipedia,


  A god card in the Yu-Gi-Oh! Trading Card Game, originally named Sky Dragon of Osiris (オシリスの天空竜 Oshirisu no Tenkūryū), was renamed "Slifer the Sky Dragon" after Slifer by 4Kids Entertainment employee Sam Murakami.


which sourced the interview with Roger Slifer himself where he answered with


  As for Slifer the Sky Dragon, despite the rumors on the web, I did not name that character after myself, even though I was a producer on Yu-Gi-Oh at the time. What happened was this: Another 4KIDS employee, Sam Murakami, was our liaison with the producer of the cards. Some names of the cards had to be changed because there was concern their names would be considered demonic or sacrilegious or something here in the states. It never made much sense to me, but when you're broadcasting here in America, it's something that has to be accommodated. Anyway, Sam renamed a bunch of those characters after people on staff at 4KIDS. It just so happened that the one he named after me happened to be a "god" card and much more prominent than the others in the series. I attribute it to the cosmic forces in the universe trying to balance things for me being made a demon in Ghost Rider. And again, just to set the record straight, I wasn't even aware of it until after the names on the cards had been changed and it was too late to go back.


This is just another case of censorship that affects some cards (also refer to an unanwered question: Was Kazuki Takahashi involved in the modifications made in Yu-Gi-Oh! for reasons of censorship?)