I have a client using French Canadian in her browser, which means the locale is French. However, the web site is English and she came across a validation problem when entering a date of US format. The validator verifies it with the French date format because the client's locale is French.
Just to clarify, you're talking about Struts server-side validations? Because it would be rather unusual to take the client locale into consideration when doing server-side validations (and I'm fairly certain that Struts doesn't do it by itself). The page should make it clear which format is required, and then the server should validate according to that format.