Thai, sometimes referred to as Siamese, is spoken in the central plains of Thailand and in Bangkok, its capital. Estimates of the total number of Thai speakers vary widely, as do the percentages of Thailand's total population of Thai speakers. Low estimates cite 20 to 25 million speakers, or about 45 percent of Thailand's population; high estimates cite about 37 million speakers, or 80 percent of the population. This includes almost 5 million ethnic Chinese who are Thai speakers and almost 500,000 speakers of Khorat, a dialect of Thai. Small Thai-speaking populations of less than 15,000 include the USA, the United Arab Emirates, and Singapore.
Thai is a member of the Southwestern subgroup of the Tai or Dai family, whose better known members include Lao and Shan.
Thai uses a script that is basically alphabetic in nature with some elements of a syllabic system. In origin, it is derived of an Indic script that was first adapted by the Khmer. There is a fairly good approximation between the Thai script and pronunciation.
Thai has borrowed heavily from Mon and Khmer. Literary Thai depends on Sanskrit and Pali, another Indic language, for much of its learned vocabulary--a situation analogous to English's dependence on Latin. Chinese has also been an important source of Thai's early loan set, contributing numerals and some few hundred basic terms. In the modern era, culinary and commercial vocabulary has also entered the Thai language via Chinese. Because many of Thai's Indic loans are polysyllabic, Thai changed from a basic monosyllabic language in its morphemic structure to one that is polysyllabic. English has also become an important source of loans, especially in the popular cultural sphere, the mass media, and commerce.
Thai is the official national language in Thailand and is used in education, the media, and government administration and bureaucracy. Most Thai people who are not native speakers--that is, speakers of the many minority languages --have at least some passive competence in the language. Official Thai policy is to promote and further Standard Thai in order to create unity among linguistic diversity. Thai itself, along with Chinese and English, is a functioning trade language. Newspapers, periodicals, and radio are mainly in Standard Thai.
Around the mid-thirteenth century, the first script, known as the Sukhotai, was developed for Thai, distinct from that of the Khmer. Modern Thai script is a modified variant of this and other intervening scripts that had been used during the reign of monarchs throughout Thailand’s history.
Source: UCLA Language Materials Project Language Profiles