All parameters (Extras, Labels) in skin.conf should be copied into weewx.conf (it is recommended)
When you have a Belchertown update, weewx.conf file is not updated.
In this case you have german parameters in weewx.conf and the same english defaults parameters in skin.conf, but the priority is for weewx.conf (in german)
If you modify other files, it is recommended to save them.
Yesterday I have made an update of Belchertown skin (from 1.1 to 1.2b5) with this method and it's work!