Probably not very far. It is not a secret that few of the second generation Sino-Thais (ethnic Chinese living in Thailand) can speak their parents' language. Some claim they can understand some, but among the 3rd generation more speak English (the semi-official language of higher education and business communication in the country) than Chinese.
However, even if you tried to locate a Chinese speaker in Yaowaraj (Bangkok's Chinatown), the chances are s/he would speak Teochew -- a Southern Chinese dialect related to Hokkien or -- less likely -- Cantonese, Hainanese or Hakka.
The historical process that has led to this peculiar situation is complex and controversial. Its major components have been:
and it can be roughly broken down into 3 distinctive stages:low Mandarin literacy intermarriages & religious similarities post-war forced assimilation
period of laissez-faire assimilation (approximately until 1938) period of state-promoted integration (from 1938 with the temporary wartime lapse) revival period (mid-80s to present)
Immigrants' Mandarin literacy
There is evidence of Chinese trade conclave communities in coastal cities of South East Asia -- similar to Venetian and Genoese trade colonies in Europe -- since time immemorial. However, Chinese immigration en mass started only in the 19th century. Low literacy level of Chinese immigrants to Thailand was due to the fact that most of them were pauperized farmers fleeing chronic famine and social perturbances. For this very reason Mandarin proficiency among the Sino-Thais is quite a recent and still limited acquisition. It came together with new affluence and still is not wide-spread as the new generation opts for the more hip and useful in business English or Japanese.
Religion and intermarriages
At the same time, thanks to religious similarities, the predominantly Mahayana Buddhist Chinese immigrants have had significantly less problems integrating into Theravada Buddhist Thailand than in any other country. In this benevolent atmosphere, intermarriages were frequent, partly owing to the lack of women among the newly arrived. The children from such alliances were more often than not brought up bilingual.
However, mild chauvinistic attitudes among the Chinese prevented them from being interested in the local culture and language more than it was necessary for business and daily communication. The perennial Chinese geopolitical and existential concept, where China -- the Middle Kingdom -- is pictured in the centre of the world surrounded by barbaric states, contributed a lot to this sentiment. Tending to congregate in compact conclaves throughout the country, Chinese were allowed to maintain their cultural identity. (This bears a striking likeness to Westerners' attitude and situation in Asia in our days.) Until now there can be found elderly Chinese who have spent most of their lives in Thailand and pride themselves on speaking no Thai.
Hard times for Mandarin
However, such Thai laissez-faire attitude came to an end when in 1938 the newly democratic Thai government launched an anti-Chinese campaign that subsided only with the WWII's onset. Chinese immigration was virtually halted, a series of measures was undertaken to reduce Chinese economic domination and promote ethnic Thai business.
It can be said that eventually Mandarin in Thailand fell a victim of the Cold War. Before proclaiming People's Republic of China, the majority of Chinese immigrants took living in Thailand simply as an opportunity to make some money and return home to savor the fruits of their hard labor abroad. However, in 1949 they were effectively barred from returning home.
Rekindled fears of pro-Communist sentiment among the Chinese diaspora prompted the Thai government to proclaim their controversial policy aimed at forceful assimilation of ethnic Chinese. It required Thai citizenship, Thai language and Thai surname as the prerequisites for entering any kind of government service, receiving any legal support or welfare. Chinese schools had to rely on community support and much later on grants from wealthy Singapore and Taiwan.
Modern situation
With the end of the Cold War, China became seen as a potential trade partner rather than an adversary. Recently, the much hyped Chinese economic boom and the ever increasing Taiwan's involvement in the Thai economy have sparked a new boom of studying Chinese. Albeit the culturally-indifferent business elite have long ago switched to Thai and English as the language of business communication, there can be seen a rapid increase in number of Mandarin courses (over 100 at present) offered in schools and universities as well as Chinese vernacular schools (126) in Thailand. It coincided with a strong tendency for re-appraisal of the Sino-Thai role in Thailand's history and culture. In addition, a number of periodicals in Traditional Chinese are printed in Thailand.
Royal patronage
Thai royalty since ancient times has been the protector of Chinese immigrants and the guarantor of religious freedom in the Kingdom. The much-loved HRH Princess Maha Chakri Sirindhorn is famous for her Chinese language scholarship and translations of Chinese poetry. She regularly presides over celebrations of Chinese New Year in Yaowaraj and visits China on state visits and study tours.
Offline reading resources
Double Identity -- The Chinese in Modern Thailand By Richard J. Coughlin Greenwood Press Reprint., 1960
The Chinese in Malaysia Edited by Lee Kam Hing and Tan Chee Beng
