Short answers, pulled from the story.
The Chinese explorer Ma Huan recorded the language of the Xiānluó kingdom in 1405. This observation was documented in his book Yingya Shenglan and noted that the local speech resembled the patois spoken in Guangdong.
Khmer-speaking captives taken from Angkor Thom after victories in 1369, 1388, and 1431 strengthened bilingualism and led to a language shift where Khmer elements transferred into Thai. Consequently, the Thai of the late Ayutthaya Period became a thorough mixture of Thai and Khmer with more Khmer words in use than Tai cognates.
Standard Thai distinguishes five phonemic tones: mid, low, falling, high, and rising. Old Thai had a three-way tone distinction on live syllables before major sound changes occurred between about 1300 and 1600 CE which split every tone into two new tones.
Since the beginning of the 20th century, English has had the greatest influence especially for scientific technical international and other modern terms. Portuguese were the first Western nation to arrive in what is modern-day Thailand in the 16th century and introduced words involving trade and religion.
Inscriptions in Thai writing began to appear around 1292 CE. The Thais adopted and modified the Khmer script to create their own writing system while the oldest known inscription in the Khmer language dates from 611 CE.