Multisite Subsite Dashboard Language

Our host is a multisite, it contains many subsites. The default language on Network is set to English.
But in some subsites, we need a different language (not a mutli-languages). So I go to Settings >> General Settings in the subsite and change the language to a different language (French, German, Chinese ...etc.). This setting works for frontend (if the theme in-used supports that language). However, the backend doesn't switch to the new language, it still stays in English. How can I change the language for the backend?