As to the written Thai language, it was based on Pali, Sanskrit, and Indian concepts, with many Mon and Khmer words entering the language.

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Thai

pcukt.com About 80% of the 60 million people in Thailand speak their national language, Thai, sometimes referred to as Siamese. Thai, which is related to languages that are spoken in Lao PDR, eastern Burma, Yunnan and North Vietnam, is considered by linguists as an "uninflected, primarily monosyllabic, tonal language" in the "Ka-Tai group." Thai. Many believe that it originated in an area that is now the border between China and Vietnam.
As to the written Thai language, it was based on Pali, Sanskrit, and Indian concepts, with many Mon and Khmer words entering the language. It was introduced in 1283 by Ramkamhaeng, the third Sukothai period king. Modern Thai readers can read inscriptions from the Sukothai era since the writing system has undergone very little change since its introduction.