I found the answer to my problem.
Solution is to read 20 times the documentation (http://www.telerik.com/help/windows-8-xaml/howtolocalizate.html) and still don't get it or simply copy/paste the following in the page constructor
Telerik.UI.Xaml.Controls.Grid.GridLocalizationManager.Instance.UserResourceMap = Windows.ApplicationModel.Resources.Core.ResourceManager.Current.MainResourceMap.GetSubtree(
where abcd is your resource file name (without .resw)
@telerik: the doc states to use InputLocalizationManager.Instance but it should be GridLocalizationManager.Instance instead. A short example would be welcome ! thx