SAI Extender is an unofficial plugin that adds features (multi-language, canvas rotation, animation timeline) to SAI Ver.1. Many artists use it safely, but it is not approved by SYSTEMAX.
Paint Tool SAI does have a drop-down menu to change languages from inside the program. The language is determined by: paint tool sai change language
To fix this, you must change the (not to be confused with display language). SAI Extender is an unofficial plugin that adds