Thanks for the questions.
Yes, as an Arab-Christian, in our family we always say "Inshallah" which means "God's Will."
God in Arabic is "Allah" and the word "Allah" has been used by Arab Christians for Centuries before Islam. So yes, both, Christian and Muslim Arabs use "Inshallah" and others words and or phrases....
By the way, my best-friend who is non-Arab (A Roman Catholic) always in writing and speaking says to me "Insh-Allah," because she understands the meaning and importance of it since she too believes in "God's Will...."