The Isan language reference article from the English Wikipedia on 24-Apr-2004
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Isan language

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Isan (or Isaan or Esarn) is the language of the Isan region of Thailand. It is a tonal language of the Tai family, and is closely related to Lao, but has also been much influenced by Thai. It was previously written using the Lao script, but is now normally written in the Thai script. The rules for determining tones vary in each dialect, but generally they closely resemble those in Lao.