In MODX 2.x, the manager language was managed through a
manager_language system setting (and optionally overridden on context or user level). In MODX 3, this setting has been removed, and the manager language is instead stored in the users' session.
When visiting the manager, it will automatically select the most likely desirable language based on the browser language. A different language can be selected at the bottom of the login screen.
To dynamically change your language while logged in, use the User > Toggle Language menu from the manager menu.