Short answers, pulled from the story.
Over 70 million people in Thailand speak Thai as of 2024. It is the sole official language of Thailand and the most widely spoken of the country's more than 60 languages by both native and overall speaker count.
Thai has five tones because the three tones of Old Thai each split into two during a period of consonant mergers that occurred roughly between 1300 and 1600 CE. When voiced and voiceless consonants merged, the former voicing distinction converted into tonal distinctions. Two of the resulting six tones later merged again, producing the current five: mid, low, falling, high, and rising.
Khmer has the deepest influence, with over 2,500 Thai words derived from it across all semantic fields, making Khmer loanwords unavoidable in any Thai conversation. Pali and Sanskrit contributed heavily, especially in Buddhist and formal registers. Since the early 20th century, English has had the greatest influence on scientific, technical, and modern vocabulary.
Ayutthaya, the capital of Thailand from 1351 to 1767, was a bilingual Thai-Khmer society. After Thai victories at Angkor Thom in 1369, 1388, and 1431 brought large numbers of Khmer speakers into Thai society, bilingualism deepened for generations before a shift occurred in which Khmer fell out of use and its vocabulary, grammar rules, and expressions were absorbed into Thai. The resulting Bangkok Thai had more Khmer words in active use than native Tai cognates.
Thai has five main registers: informal street Thai used between friends and family; formal written Thai used in newspapers and official documents; rhetorical Thai for public speaking; religious Thai influenced by Sanskrit and Pali; and Royal Thai influenced by Khmer for addressing members of the royal family. Most Thais can speak and understand all five registers.
The Royal Thai General System of Transcription, published by the Royal Institute of Thailand, is the official standard for writing Thai in the Latin alphabet. It is used on road signs and by Thai government bodies, but it does not mark tone or vowel length, and the original Thai spelling cannot be recovered from it. A separate ISO standard published in September 2005, ISO 11940, uses diacritics to make the transliteration fully reversible.