Well, it's a fact, more people in thailand speak one of the Isaan dialects (which there are a LOT but primarily 6 or 7) in their home than speak central thai.
Now be careful throwing out there that Isaan thai is a "Lao dialect"! You'll sure piss off the 6+ million born-bred-rice fed Laotians who speak "real Lao".
To a person they think ANY of the Isaan dialects are just bastardized Lao
. Also EVERY Isaan thai version is written in THAI script, not Lao script..
This question has come up on almost every thai language related forum out there. What it boils down to is;
EVERY SINGLE thai in this country who EVER attended school learned to speak, understand, read & write Central Thai (the government approved version of the language), period, end of story. I'm not saying when those kids got outta school and went home they didn't speak their local Isaan version with their friends, their families, etc. I'm stating a fact that all thaiz who attend school learn ONLY Central Thai.
This means that almost to a person, any thai you talk to is able to converse and understand central thai (even a piss-poor foreign accented version).. To me, it means that Central Thai is the way to go to learn to communicate with thaiz the country over.
Isaan ain't gonna get you very far in the south, or up in the north west (Chiang Mai, Chiang Rai etc).. BUT Central Thai will work in both places just fine.
I run into foreigners all the time who say, "I have a gurl friend from Isaan, so I wanna learn Isaan thai." Well, there's NO schools teaching that version of thai out there. It's all gonna be pretty much self taught.
Not to mention the Nong Khai version, the Ubon version, the Buriram version, the Surin version, the Korat version and a couple other "versions" of Isaan thai are distinctly different. For the most part they're mutually intelligible but there's NOT just "Isaan Thai", like there is Central Thai.
Now while farang84 says he's looking for a logical discussion I might say, "Don't go looking for logic where there is none to be had; especially where the thaiz and this country are concerned".
Almost to a person, ANY Bangkokian is going to look down on someone speaking Isaan. I'm not saying it's right, I'm saying it is what it is.
. Wearing rose colored glasses and pretending it ain't so doesn't change that fact a bit..
I'd say you're going to get far more mileage out of Central Thai than you will any of the Isaan Thai "versions". If you wanna learn some Isaan Thai, I'd recommend getting Bemjawan Poomsan Beckers C/D work book called "Speak like a Thai Volume 5 Northeastern Dialect http://www.paiboonpublishing.com/details.php?prodId=64
That will at least get you started, and you can go from there.