The official language of Vietnam is Vietnamese, a member of the Austro-Asiatic language family. Linguists usually consider Vietnamese to be a distinct language group, although it has some similarities to Chinese and other languages spoken in Southeast Asia. Like Chinese, Vietnamese is a tonal language, but its syntax is closer to Khmer, the official language of Cambodia. Other languages spoken in Vietnam are Chinese, Khmer, Cham, and various tribal languages spoken by peoples living in the mountains. When China conquered the Red River Delta in the 2nd century bc, Chinese was adopted as the official language. Eventually a separate script based on Chinese characters and known as chu nôm (southern characters) came to be used unofficially for the written form of Vietnamese. In order to translate works of scripture, Catholic missionaries devised a form of written Vietnamese using the Latin (Roman) alphabet in the 17th century. This system, known today as quoc ngu (national language), was the first to indicate tones through the use of accent marks. In 1910 quoc ngu officially replaced Chinese characters as a means of writing Vietnamese, and in 1954 the governments of both North and South Vietnam adopted it as their national script.