If you are a business, this is the top legal source. Office 2013 Professional Plus Volume License customers have access to the Thai Language Pack as a downloadable ISO.
Office 2013 comes in 32-bit and 64-bit. You must match the language pack to the version of Office installed. office 2013 thai language pack top
Once you have acquired the ISO or setup file (typical filename: mu_office_2013_language_pack_thai_x86_x64_4731695.exe), follow these steps to avoid UI corruption. If you are a business, this is the top legal source