Isan (Thai: ภาษาอีสาน, RTGS: phasa isan, IPA: /pʰaːsaː iːsaːn/) is the principal language of the Isan region of Thailand, which lies in the country's northeast. A tonal language of the Tai language family, it is the main language of trade and communication in the Isan region, except for in cities and in media where it gives way to Thai.
The language is a dialect of the Lao language spoken in the neighbouring country of Laos. This is because Isan was historically once a part of the Kingdom of Lan Xang, which was a Lao Kingdom, and also because large forced population transfers from Laos to Isan were undertaken at various points in history. In fact, the two dialects are almost identical, with differences contributed mainly by neologisms created after the two areas were no longer a single political entity and modern Thai began to contribute to the Isan vocabulary. The language is still sometimes occasionally referred to as Lao, either by older Isan people or by Thais pejoratively. Isoglossic differences in Isan mirror those on the other side of the Mekong, so that a speaker from Nongkhai sounds more like a speaker from Vientiane than he would with a speaker from Ubol, who may sound..... full article at Wikipedia