after xls(contains english values and Thai values) saved as csv file, Thai value turn to ?

when I was trying to save a xls document(contains Thai values and english values) to csv in excel 2010. All the Thai values turn to question mark.
After search a bit in the google,I found some posts advice me to save as unicode plan text first, then save the plan text to ANSI text then open the ansi text in excel.
However when I in the step to save unicode plan text to ansi plan text in the note pad, there is warning message says 'This file contains characters in Unicode format which will be lost if you save this file as an ANSI encoded text file'

Please kindly advice the solution.

Thanks

please let me know if you need more detail information,thanks