Changing the backend language
By default, TYPO3's backend is set to English with no additional languages available.
Table of Contents
Set your backend language
One of the available backend languages can be selected in your user account.
Go to Toolbar (top right) > User Avatar > User Settings and select the new language from the field Language:

If the desired language is not available, administrators with system maintainer permissions can Load an additional language pack in the Admin Tools.
Save the settings and reload the browser content.
Note
This change only applies to the currently active account.
Change the default backend language of a backend user
As an administrator you can change the backend language of another user.
In the record of the backend user, tab general, choose "User interface language".
This setting only takes effect for users that did not switch the backend language in their user settings.