The system has two levels of language settings for the interface language.
System-wide Interface Language – this is the default language that every user sees. This can be set by an administrator
Step 1: As an admin, you can set the system-wide default for all users by going to Admin -> System -> Options -> Language -> Interface Language
Personal Interface Language – this is the language that any individual user can set for themselves.
Step 1: Each user can override the system-wide setting to set their own interface language under Admin -> Personal -> Options -> Language -> Interface Language