This week's book giveaway is in the OO, Patterns, UML and Refactoring forum. We're giving away four copies of Refactoring for Software Design Smells: Managing Technical Debt and have Girish Suryanarayana, Ganesh Samarthyam & Tushar Sharma on-line! See this thread for details.
There is not a way to do this with the "out of the box" validation rules. However, the struts Validation framework was created with extension in mind, so the solution would be to create your own validation rule that validates only for certain locales.
Here is a good article on the validation framework. Look in the section entitled "Creating Your Own Validators" for information on how to extend the validation framework.