From Thai Language Wiki
Standard Thai, the subject of this reference grammar, is the national spoken language of Thailand and at the same time an educated variety of the dialect of Bangkok, to some extent also of the entire Central Plains region of the country. As such it is the most widely known representative of the Tai language family, which extends from southern China to the Malay peninsula and includes present-day speakers in all the countries of Mainland Southeast Asia. Standard Thai, along with its written counterpart, is known to some extent to nearly all the people of Thailand, and it enjoys some status as secondary language in neighboring countries as well. As is the case with many national languages, it is difficult to state the actual number of native speakers. The usual estimate of 18,000,000 is probably accurate.
The linguist responsible for all phases of the reference grammar project has been Richard B. Noss. The present work is a greatly revised and expanded version of his dissertation, 'An Outline of Siamese Grammar,' Yale, 1954. The original research was made possible by concurrent grants from the Yale Southeast Asia Program and the American Council of Learned Societies. The revision and publication was supported by the U.S. Office of Education through a grant to the Foreign Service Institute. Some of the additional research on which this revision is based was done in Thailand in 1961, when Dr. Noss was serving as a Regional Language Supervisor in Southeast Asia for the Foreign Service Institute.
Names of principal informants consulted for the study will be found in the Introduction (p.3). They include two other FSI staff members: Mr. Prasert Crupiti and Miss Chotchoi Kambhu.