Actually, there's an even easier solution.
Set up the LocaleResolver and LocaleChangeInterceptor as you described above, but also define a Spring MessageSource like so:
and the files with the appropriate suffixes (messages/errors_es.properties, messages/errors_fr.properties) will be loaded and referenced without any other effort on your part. This integrates seamlessly with the JSTL fmt:message tag as well, so you can use the "standard" approach to access your internationalized strings.
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Hope this helps