How to reproduce:
1. Set your !!! language to Russian RU or another which is located below the German DE (depends on step #3)
2. Go to Users page and change for example Guest's language to German DE, press Save. At this step, locale settings correctly saved to DB - verified.
3. Open again Guest's settings - you will see Russian RU language (depends on step #1) instead of German DE.

I.e. If you select and save the user language, which located higher in the list than your current language, then instead of his actual language frontend shows your current language.
If you save the user languages that is below your language - no problems with displaying user's language.