Issue a warning if /system/locale/region points to a unknown locale

[Michael]

After upgrading from 14.04 to 16.04, I noticed that several programs got problems. The problem was that some of the LC_ was put to a non-existent locale (CH_zn in my case) The cullprit was the dconf setting " /system/locale/region" I didn't found a setting in the gui (including system settings -> languages -> regional settings, which was already set to german). I fund the settings only by a raw filesearch. So there should be a warning during starting, and the language setting tool shoud allow to set this setting as well. I file it to dconf because it is a setting in dconf but change it at your will.