The word Allah is not specific to Islam. It's an Arabic word for God. But it is frequently used in the form of a proper-noun to describe a god specific to Islam, when in fact Arab Christians and Jews also use it since, in their language, it's the word for god.
So why does it get used in English so much?
If one were to translate Deus é misericordioso from Portuguese, for example, one would write God is merciful, and not Deus is merciful. Surely the same consistency must be applied when translating from Arabic?
What if the source were a Christian Arab? Would الله be translated as "Allah"?
There are other examples: The word madrassa means school in Arabic. Yet it is used to describe an extremist religious school. I went to a madrassa, and so did most students in Arabic speaking countries, whether they were Christian, Jews, etc...
Even though a person uses the word Allah or God, the person has its own understanding on that word he is referring to. Certain muslims could speak about Allah and by that giving "him" (see even the word him is used because of our limited language is forcing us too) attributes hands, feet, eyes, place and so on. We ourself differ on the understanding of the word Allah.
– Kilise Dec 30 '15 at 16:16