Overhaul localizations + add dutch

• Split localizations over multiple files to make it more structured
• Get preferred language from the SSO server
• Allow users to configure preferred language
• Add logic to switch language to the correct one
• Add notification setting explanations
• Add notification setting targets to hide settings for users where the notification does not apply
• Add dutch localizations (Dutch is currently not enabled as an available locale, but will be in the future)