Hehehe! Thailand has ethnic Khmers, Shan, even some Cham, and the largest Lao population in the world (quite a bit more than Laos, mainly in the Isan region). Come to think of it, the Royal Family has Mon antecedents, and Taksin the Great (who was deposed by the current dynasty) was ethnic Chinese.
And then, Ayutthaya Thai and Lanna Thai aren’t quite the same people.
I’d say that ethnicity/nationality questions can be particularly vexed in Thailand. 
