Language
Spoken and written Thai remain largely incomprehensible to the casual visitor. However, English is widely understood, particularly in Bangkok where it is almost the major commercial language. English and other European languages are spoken in most hotels, shops and restaurants, in major tourist destinations, and Thai-English road and street signs are found nation-wide.

Standard Thai, also known as Central Thai or Siamese, is the official language of Thailand, spoken by about 25 million people (1990) including speakers of Bangkok Thai (although the latter is sometimes considered as a separate dialect). Khorat Thai is spoken by about 400,000 (1984) in Nakhon Ratchasima; it occupies a linguistic position somewhere between Central Thai and Isan on a dialect continuum, and may be considered a variant or dialect of either.

In addition to Standard Thai, Thailand is home to other related Tai languages, including:

Isan (Northeastern Thai), the language of the Isan region of Thailand, is considered by some to be a dialect of the Lao language, which it very closely resembles (although it is written in the Thai alphabet). It is spoken by about 15 million people (1983).
Nyaw language, spoken mostly in Nakhon Phanom province, Sakhon Nakhon province, Udon Thani province of Northeast Thailand.
Galung language, spoken in Nakhon Phanom province of Northeast Thailand.
Lü (Tai Lue, Dai), spoken by about 78,000 (1993) in northern Thailand.
Northern Thai (Lanna, Kam Meuang, or Thai Yuan), spoken by about 6 million (1983) in the formerly independent kingdom of Lanna (Chiang Mai).
Phuan, spoken by an unknown number of people in central Thailand and Isan.
Phu Thai, spoken by about 156,000 around Nakhon Phanom province (1993).
Shan (Thai Luang, Tai Long, Thai Yai), spoken by about 56,000 in north-west Thailand along the border with the Shan States of Burma (1993).
Song, spoken by about 20,000 to 30,000 in central and northern Thailand(1982).
Suay Language, spoken around Surin province. It is a mixture of the Thai and Khmer languages.
Southern Thai (Pak Dtai), spoken about 5 million (1990).
Thai Dam, spoken by about 20,000 (1991) in Isan and Saraburi province.
Statistics are from Ethnologue 2003-10-4.

Many of these languages are spoken by larger numbers outside of Thailand. Most speakers of dialects and minority languages speak Central Thai as well, since it is the language used in schools and universities all across the kingdom.

Standard Thai is composed of several distinct registers, forms for different social contexts:

Street Thai: informal, without polite terms of address, as used between close relatives and friends.
Elegant Thai: official and written version, includes respectful terms of address; used in simplified form in newspapers.
Rhetorical Thai: used for public speaking.
Religious Thai: used when discussing Buddhism or addressing monks.
Royal Thai: used when addressing members of the royal family or describing their activities.
Less-educated Thais usually can speak at only the first and second levels, though they will understand the others.