Quite often it's the Lao language.
Sometimes it's Khmer, Mandarin or Arabic.
There are 71 different languages in use in Thailand.
Only 40% of the Thai population are native Central Thai speakers.
https://en.wikipedia.org/wiki/Languages_of_Thailand
Now if you want to claim they are all minor variations of the same language, Kra-Dai, then you probably believe that most of Europe also speak the same language, variations of Latin.