The La Chi speak a Thai-Kadai language.  Specifically, it is in the Ge-Chi subgroup of the Kadai branch of the language family.   Their language is a completely different language group from Vietnamese, which is of the Mon-Khmer group.  

It is closely related to the Gelao (Co Lao) language, linguistically, though the two are not mutually intelligible.  A lexical study of the La Chi language found that it has 36% similarity with Gelao, 33% similarity with Pu Peo (Pubiao) and 28% similarity with  Zhuang. It is fast becoming extinct in Vietnam, though 70% of the La Chi in China can still speak the language.  The areas in which the La Chi live are multi-ethnic and multi-lingual.  As a consequence, only a few of the old La Chi are still able to speak their language.

     Ropal code for the La Chi language: LBT00 and LWH00