It seems there are some missing entries in the default localization (at least for the german localization) for the grid in the FilterRow mode and a boolean column for languages other than english.
The filter popup shows "True" and "False", though everything else is correctly localized.
The solution was to add the entries 'GridViewFilterIsFalse' and 'GridViewFilterIsTrue' to the localization resource, maybe this is a help for someone else with the same problem.
see:
http://docs.telerik.com/devtools/wpf/controls/radgridview/filtering/how-to/howto-localize-the-filtering-ui.html and
http://docs.telerik.com/devtools/wpf/common-localization.html