Dear Mannus,You would need to customize default template of EditorWidget control and replace text strings with translated values. Here is what you can find in the EditorWidget template (for your case you should modify the part with TooltipService.Tooltip="Add to selection", etc...):
...
<Button x:Name="AddSelect" Command="{Binding Select}" CommandParameter="Add" ToolTipService.ToolTip="Add to selection" Style="{StaticResource ButtonStyle}">
<Image Stretch="None" Source="/ESRI.ArcGIS.Client.Toolkit;component/Images/AddToSelection.png"/>
</Button>
...