Oh by the way, another coda here:
Arabic-speaking Christians pray to Allah, and call Jesus "Isa". Middle Eastern Muslims also pray to Allah, and also refer to "Isa". English speaking Christians pray to God. In France, they pray to Dieu. Your own particular prejudice of birth and culture is immaterials in determining the faith of a particular speaker. Jews typically do not write the proper name of God in Hebrew, which is spelled with the Hebrew characters yud, heh, vav, and heh. You often hear Jews use the name "Adonai," which means "sovereign" or "lord," but is more a rank than a name. (Other biblical sobriquets are similarly not considered THE name of God, but are ways to refer to God -- so they are names but not the proper name.)
So the notion that Warren chooses to use the Arabic name of Jesus does not make a lot of doctrinal difference. If you believe that Christianity is a form of monotheism, and the minority of the world that is Christian does believe that, then Rick Warren has not introduced polytheism by using "Isa" as a name for "Jesus," and would furthermore not be a Muslim because he uses the Arabic "Allah," any more than he is a Germanic pagan because he uses the Northern European "God."
PFnV