@Gra***
keep in mind there is only ONE government approved version of the thai language and that's Central Thai.
If a thai spent a day in school they had that language beaten into their head, it's spoken at ALL the schools, the government offices and on all the t/v stations.
I can ONLY speak Central Thai and I traveled the length and breadth of this country from Chiang Rai to Hat Yai, Chantaburi to Kanchanaburi and was understood and replied to in kind by everyone.
There is next to no value in learning one of the many many dialects here because it's useful ONLY in one area where as central thai can be used everywhere you go.
As far as the Isaan dialects, someone from Nong Khai (that has a dialect closer to real Lao) might have a problem understanding someone from Surin (that has a dialect with a lot of Khmer words in it). There is NO one isaan language, there's a whole bunch of loosely related ones. NONE of them are pure thai and none of them are pure lao.