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:
To update translation(s):
A message which displayed if the version of your language is already up to date, if not then the update will be downloaded.
To complete this step, a restart of the application is required.