Lao people, or Laotian people consider ourselves different from Thai people. We strongly believe that our culture, our food and our language is different to Thais and we want to preserve it.
The mainstream Lao people don't read, write, or speak Thai language in their daily lives, nor are we taught Thai in school but we understand their language naturally because Laos receives Thai television and radio broadcasts.
The younger generation of Lao people also enjoy listening to Thai music and watching Thai movies, and frequent crossing the border into Thailand for shopping, visiting relatives, medical care, business and employment reasons makes Laos considerable exposure to the Thai language.
Most Thai people in the Central Region of Thailand don't understand Lao language because there's probably no need to learn it. Another reason is that Thai people are quite weak in foreign languages, just observe yourself, most Thais can't even speak English properly. Lao people tend to pick up the English language very fast. Sadly, even Thai people who have lived and worked in Laos for a very long time are not fluent in Lao language, so Lao people need to talk Thai with them.
The native people of Northeastern Thailand are Issan people who are ethnic Laotians so they know how to speak Lao or "Thai-Issan" language. However, being Thai nationals they are required to speak and learn Thai at school which is the national language of Thailand.
Thai language is a difficult tonal language that uses alot of tongue twisting and it can be pretty daunting for new learners, but Lao people don't have any problems communicating with a Thai person in their language.