Language of Thailand

Talk
The official language of Thailand is Thai. There are dozens of small language groups in the tribal areas of the north, and a small number of places where Thai speakers are few and far between. Thai is a tonal language (think about the difference in your voice when saying "yes." versus "yes?" - that's tonal) which can make it tricky for Westerners to learn quickly, but despite this, everyone will appreciate any attempt you do make so pick up a phrase book and give it a go. In the Muslim-dominated south, dialects of Malay that may be somewhat incomprehensible to speakers of standard Malay or Bahasa Indonesia are spoken.
Public signage is generally bilingual, written in both Thai and English. There is also some prevalance of Japanese and Chinese signs. Where there is English, it is usually be fairly phonetic - for example "Sawatdee" (meaning hello) is pronounced just as it reads: sa-wat-dee. There is no universal agreement on how to transcribe Thai letters that don't have an English equivalent, so Khao San Road for example is also commonly spelt Kao Sarn, Kao Sahn, Khao San, Koh Saan, Khaosan, and many other variations. Maps with names in both Thai and English make it easier for locals to try and help you.
Most "front desk" people in the travel industry speak at least enough English to communicate, and many are relatively fluent; some also speak one or more other languages popular with their clientele, such as Chinese, Japanese, German, etc.
Many Thais have trouble pronouncing the consonant clusters of the English language. Common confusion comes from the fact that Thais often pronounce "twenty" as "TEH-wen-ty", making it sound like they're saying "seventy". Therefore it is a good idea to make use of the calculators that street vendors may offer you in order to avoid confusion about prices offered when buying goods.