Note: This is the only official method. If you have a pirated copy or an old repack, this won't apply to you.
Most users looking for "paint tool sai change language" are using SAI Ver. 1 (the classic version with the grey interface). Over the years, fan-made repacks (like SAI 1.2.5) have included multiple languages hidden in the installation folder. paint tool sai change language
Here is the most common fix for SAI Ver. 1: Note: This is the only official method
The method differs significantly between version 1 and version 2. Most users looking for "paint tool sai change
After reboot, launch SAI. If the menus are still foreign, you must also reinstall SAI after changing the locale so it reads the correct language strings during installation.
If you are using the modern version (Ver 2), the developers have streamlined the process, assuming you have the correct translation files installed.