May someone enlighten me about northern & Isan- languages ?

What R the differences and how do the 2 languages R related to Lao-Vientiane language ?

How many speakers do the 2 languages hv ?

Wikipedia has a decent article under "Isan." It also has good references on most languages including the various Tai languages.

In short, Isaan, the language, is closely related to Lao, and might be considered one of the Lao dialects. It is a more difficult argument, and ultimately becomes a political argument, that Isaan is a dialect of Central Thai, both from a linguistic and historical perspective.

It has been decades since I read up on this, but I believe it was Marvin Brown, of AUA fame, who postulated that there were several distinct migrations of Tai into Southeast Asia from southern China accounting for the different Tai languages in modern Thailand. An early migration included the Shan, who were seen as the larger group by following migrating Tai groups who then referred to the established Shans as the Tai Yai. Other migrations are postulated that led to the development of Southern Thai, Cental Thai, and the northen languages.

Northern Thai, Kham Muang, from my subjective view, is closer to the Lao langauges than it is to Central Thai, but is not a dialect of either of those two Tai languages. Not only are the Lao-Isaan-Khon Muang closer to each other linguistically, but the speakers share some cultural attributes that distinguish them for the Central Thai, the most prominent being the preferred rice they eat, sticky rice.

Wikipedia has a decent article under "Isan." It also has good references on most languages including the various Tai languages.

In short, Isaan, the language, is closely related to Lao, and might be considered one of the Lao dialects.

Or, given the population numbers, that Lao is an Isaan dialect!

Northern Thai, Kham Muang, from my subjective view, is closer to the Lao langauges than it is to Central Thai, but is not a dialect of either of those two Tai languages. Not only are the Lao-Isaan-Khon Muang closer to each other linguistically, but the speakers share some cultural attributes that distinguish them for the Central Thai, the most prominent being the preferred rice they eat, sticky rice.

Historically it does seems that Kam Mueang shares an ancestry with Central Thai rather than with Lao. However, in more recent history it has closer connections with Lao. The shared features criss-cross - the old voiced stops (ค ท พ) became voiceless aspirates in Central Thai and Lao, but merely voiceless stops in Kam Mueang, with consequent similarities in tone splits in Central Thai and Lao. (The U Thong dialect of Central Thai preserves the 3-way tone split between high, mid and low consonants in live syllables without tone marks - a.k.a. old tone A - also found in the 6-tone accent of Vientiane.) Kam Mueang and Lao seem to share the develoment of /ch/ to /s/, but I am not sure of the Kam Mueang outcome - possibly, as in Shan, the development of /ฉ/ is not the same as that of /ส/, where as it is a complete merger in Lao. The change of /ร/ to /ฮ/ is common to all *living* SW Tai dialects outside Thailand - Central and Southern Thai are exceptional in resisting this change. (Literary Tai Lue seems to be dead - the New Tai Lue script seems to make no provision for the retention of any sort of /r/ distinct from /h/.)