There should be not problem to bind ErrorToolipContent property of RadDateTimePicker. Regarding localization - I would suggest to use LocalizationManager and create a separate .resx file for each one of the languages your application will support. The approach is described in details here. I've also prepared a simple example, please download the attachment and examine it.
Hope this helps.
the Telerik team
Do you want to have your say when we set our development plans?
Do you want to know when a feature you care about is added or when a bug fixed?
Telerik Public Issue Tracking
system and vote to affect the priority of the items