That is not a validation error message, it is a conversion error message.
You can override the default conversion error message up to each single object, by creating an entry for it in the global .properties file, as described in Struts 2 documentation, Type Conversion Errors Handling:
By default, all conversion errors are reported using the generic i18n
  key xwork.default.invalid.fieldvalue, which you can override (the
  default text is Invalid field value for field "xxx", where xxx is the
  field name) in your global i18n resource bundle.
However, sometimes you may wish to override this message on a
  per-field basis. You can do this by adding an i18n key associated with
  just your action (Action.properties) using the pattern
  invalid.fieldvalue.xxx, where xxx is the field name.
If you are interested in understanding how it works in a deeper way, read the Short Story about Validation, Conversion and Friends.