@aerendir I can reproduce this error. The LanguageHelper in the error message triggered me to set Mautic to a different language than the default (default is English US if I remember correctly). That indeed triggers the error.
When I set the Mautic language to its default (English US) the update succeeds without issues:
denni@mautic2160-web:/var/www/html$ app/console mautic:update:apply
Are you sure you wish to update Mautic to the latest version? y
Step 7 [============================] Success! You are now running version 2.16.3 of Mautic.
Can you confirm if that solves the error for you as well? If it does, I’ll check if this bug is still present in Mautic 3 and so if we need to re-open your GitHub issue. 