Set messagesUrl before Language on ready

[matthewlawson]

Using this strategy to setup localisation the url will not be set correctly in the first _fetchLanguage call.

<script>
document.addEventListener('HTMLImportsLoaded', function() {
    I18nMsg.url = "/locales";
    I18nMsg.lang = 'es';
});
</script>

This pull request sets language after the messagesUrl so that when the languages observer is triggered the messageUrl will be correct and load the correct translation file.

[ebidel]

LGTM. Thanks.