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...