@rfellows now "common-ui/jquery-i18n" depends to my rewritten plugin -> all code in the project will use it. If I'll return default browser locale, 404 errors would happen again (default browser locales will be downloaded ignoring results of _supported_languages.properties). With my fix if we didn't specify the locale and default browser locale wasn't find in _supported_languages.properties, no extra requests will be sent except messages.properties.