After installation, you need to set Thai as the active language:
Are you using Microsoft Office 2013 but need to work with Thai scripts, fonts, or a localized interface? While Office 2013 is an older version (end of support), many businesses and individuals still rely on it. Adding the Thai Language Pack can help you display menus, help files, and proofing tools in Thai. office 2013 thai language pack
You must have a valid, installed version of Office 2013 (Professional, Home & Student, etc.). After installation, you need to set Thai as
Changes all menus, ribbon tabs, dialog boxes, and tooltips into Thai text. installed version of Office 2013 (Professional
Missing Windows Thai font support or corrupt font cache. Fix: