Localizing error messages
By default, platform-generated error messages are localized in the language that is assigned to the logged-on user.
Validation rule messages are mapped to both field labels and custom labels. Error messages that are mapped to field labels can be localized using the Translation Workbench. For more information, see Localizing field labels. Error messages that are mapped to custom labels can be localized on the custom field details page. For more information, see Localizing custom labels.