Thailand and Cambodia are both diverse societies, linguistically and culturally. In Thailand, the official language is Thai. Unlike most European languages which use the Roman/Latin alphabet, Thai uses a completely different script.

https://www.omniglot.com/writing/thai.htm

Unlike English and the other Romance and Germanic languages of Europe and North America, Thai is also a tonal language. This means that every syllable and work is pronounced in 5 different ways with 5 different tones: low, middle falling, high and rising.  In English we might inflect a bit in our pronunciation for effect but in Thailand this changes the entire meaning of the word.

http://thai-tones.com

In Cambodia, the official language is Khmer (pronounced k-my). Again, Khmer language uses it’s own script, totally different from the Thai script.

https://www.omniglot.com/writing/khmer.htm

Although there are a few sounds and terms that are similar in both languages, they are for the most part, mutually exclusive. Few Cambodians speak Thai unless living near the Thailand border, and even fewer Thais speak Cambodian.