Hi,
I think first you need to copy language resource file to CMSResources then in Kentico Admin go to Configuration > Localization > Cultures. Select culture and check inside Is UI culture.
Note also that resource file will be names with culture code. Example: CMS.de-de.resx (german), CMS.hr-hr.resx (croatian ), CMS.sk-sk.resx (Slovak)... etc.