MuseScore works with your "System" language (the one used for most programs, and generally depending on your country and the language settings of the PC, or account).
In the General tab, select the desired language from the drop-down list in the Language section:
The Resource Manager is used to install and uninstall extensions, and to handle the update of translations. To access the menu use one of the following options:
From the menu bar, select Edit→Preferences... (Mac: MuseScore→Preferences...), open the General tab, and click on the Update translations button.
To install or uninstall an extension:
Note: Extensions currently include the MuseScore Drumline (MDL) (as of version 3.0) and the MuseScore General HQ soundfont (as of version 3.1).
To update translation(s):
Note: Almost all menus and dialogs will switch to the changed/updated translations immediatelly, but some won't use the them until the program is restarted.