The Multilingual User Interface (MUI) functionality in Microsoft SharePoint gives users the ability to change the language of the user interface (UI) for a website. For example, a German-speaking user who is working with an English UI can change the language of the UI to German. The site's toolbars, navigation bars, list titles, and column headings appear in German.
After you run the pre-upgrade site collection health check, if the health check report shows a list of sites within your site collection with unsupported MUI references, you will not be prevented from upgrading your site collection. You will just need to correct this issue before users will be able to view the sites within the site collection in the appropriate language.
To fix this issue, do one of the following:
- If you’re using SharePoint Server 2013, contact your Farm Administrator to get the missing language pack installed and enable the language as an alternative language for the site.
- If you’re using SharePoint Online 2013, you will need to wait for the missing language pack to become available before you can enable the language as an alternative language for the site.