In my opinion, Chiang Mai (which is bound to get suggested) is highly overrated and absolutely saturated with tourists. My girlfriend and I spent almost 2 years living and teaching all over Thailand so we absorbed a little bit of language, culture and etiquette ('maliart' in Thai) each day.
The children at our school received lessons in 'maliart' and there are national competitions each year but insofar as a foreignor learning maliart Thai, I am not aware of any classes. If you want to immerse yourself in Thai culture here are a few recommendations:
- Take a cooking class: Food is one of the most important aspects of Thai life
- Take a Muay Thai class: A unique (and tiring experience). Its very easy to find very high-quality tuition and you can book a one-on-one class quite easily.
- Visit a Thai market during a festival: See live Thai music, wander round and sample Thai snacks, laugh at the Thais getting drunk and dancing like fools!
- Spend a bit of time in any major city during Songkhran (Thai New Year in April): In Chiang Rai, we found ourselves drinking Sangsom amongst thousands of other Thais partying the night away with karaoke and dancing
- Learn Thai massage
- Take a Thai language class
- Vist a Thai karaoke bar
Anything I've missed out?