i say 'God' when i pray in english and 'Allah' when i pray in arabic.
of course, i use 'Ilah' in the appropriate grammatical parts of the prayer and in the nicene creed (which is discussed in a previous post).
there are subtle differences (to a non native speaker of arabic; probably big differences to a native speaker) between Christians and muslims when they discuss their beliefs in arabic.
you can generally tell by the context if the arabic speaker is Christian or muslim.
arabic speaking Christians are not at all easily confused with muslims (except by those sort of people who ignore the cross around the neck and references to going to church, and assume that fasting plus speaking arabic equals muslim!)
have to go now, time to pray (yes there will be a little arabic used!)