1. NT Greek is a 'common man's Greek--it was the ONLY language spoken throughout the world at the time. Scholars spoke a 'higher Greek' generally, so this language choice was for the purpose of mass/wide dissemination of the material. (Alexander the Great had literally FORCED all his conquered lands to adopt this Common/Koine Greek).

2. Most people in metro areas or commerce spoke at least 2 languages: their local tongue (e.g., Aramaic, Ethiopic, Persian) AND common-Greek (to do business and government). It was sorta like a requirement to do business at any scale. The NT authors would have been able to speak it, but some probably could not write it very well (but many of them would have been able to: Paul, Matthew, Luke, Author of Hebrews, Mark--from what we know about their backgrounds and style), but scribes were always available (for a fee!) for helping them with the manuscript writing anyway. Frequently people dictated works to others who took the dictation, and wrote out the content--this was common praxis at that time, and was often used by those who COULD write well. We have many, many cases in which a famous author--able to write fluently--used household servants to do the writing, while they composed and dictated orally.