Language spoken along the Mekong (4)
'language spoken along the mekong' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for lao that I've seen before include "Indo-Chinese language" , "Asian" .)
(Other definitions for thai that I've seen before include "From Bangkok, for example" , "Easterner" , "nationality" , "SE Asian" , "Asian language" .)