the Fatwa Department Research Committee - chaired by Sheikh `Abd al-Wahhâb al-Turayrî
Yes, you may use the word “God” – with a capital "G" – when speaking English, especially when you are speaking to non-Muslims. In some cases, it is actually a good idea to do so, since it makes it clear to people that we are talking about the one and only God who created everything and not some other entity.
Some non-Muslims are under the misconception that Allah is some particular "Muslim God" and not the Creator of the Heavens and the Earth. Some religious groups encourage this misconception among their followers to keep their people away from Islam. When Muslims are too insistent upon using the name "Allah" with non-Muslims, they can unwittingly reinforce that misconception.
The word Allah comes from the Arabic word “ilâh” meaning “deity, god, divine being” combined with the definite article “al”. Put together, they make “al-Ilâh” or literally “the God” which was then brought together as “Allah”. The name therefore, literally means "the One and Only God".
It is, of course, good to refer to Allah by His name – Allah – since this is how Allah refers to himself in the Qur'ân. Also, the name Allah is clear in asserting the uniqueness of God, since there is no way the name "Allah" can be made plural or changed to indicate gender, as is the case with the word "God". However, it is wrong to be dogmatic about this issue, especially when it leads people to have misconceptions about Islam. Insisting upon the use of "Allah" at all times and refusing to use the word "God" can give non-Muslims the mistaken impression that we worship a different God than the God of Abraham, Moses, and Jesus (peace be upon them all) or that we worship other than the Creator of the Heavens and the Earth.
Therefore, we can declare without hesitation: "There is no god but God and Muhammad is the Messenger of God". When we do so, we are speaking the truth – and what we are saying will be understood by non-Muslims.
And Allah knows best.
Source: Islam Today