When opening a page in explicitly defined content language, the Sitecore user is redirected to the page in the default language of the Sitecore Client defined in the ClientLanguage setting. For example, if the user navigates to /de?sc_mode=edit URL in German, they get redirected to the /de?sc_mode=edit&sc_lang=en in English.
To resolve the issue download and install the appropriate patch available on this page: https://github.com/SitecoreSupport/Sitecore.Support.118411/releases.
To get a solution for other Sitecore XP releases, contact Sitecore Support.