Mae Sai is a border town. Like all border towns everywhere else in the world, it sees its fair share of immigrants.
However, in recent times, the number of immigrants have exploded to the point that it seems that there seems more of them than incumbent Thais.
In Mae Sai establishments such as clinics, post offices, stores etc. you will find dual-language signage. Thai and Burmese language.
Most of the immigrants are either Burmese or Tai Yai. Although Tai Yais hail from the Shan state in Myanmar, and so are technically Myanmarese, they are different and not only just in appearance. Different language, culture, food, clothing etc.
One way you can tell is by clothing. Burmese men wear a sarong-like cloth called the lungyi.
Burmese women put a yellow powdery substance called Thanaka (from the bark of a tree) on their faces to beat the heat. This is different from northern Thais and Tai Yai who merely use talc. Not my photo but this shows an example
A Tai Yai women in traditional dress. She is probably in town to make tamboon - they dress up to go to the wat as people used to do in the west to go to church.
Tai Yai male in traditional wat-going dress
Burmese nuns
And how do they come from Tachilek. A common mode of transport is the Burmese tuk-tuk which is different than the Thai version. It is significantly bigger for one and uses a bigger bike. Thye smaller ones (as here) seat 12 and the larger ones seat up to 18. You can tell it's Burmese by the license plate as well.