If possible, the interface should be translated in the preferred languages send by the browser. At the moment, the main language of the matched siteaccess is used with a fallback on "en".

This involves several changes:

1. shell.html.twig should load the translations configured in the browser
2. Y.eZ.trans() should be more than a shortcut to Translator.trans(), it should iterate over the languages to find the best translation
3. the same strategy should be applied to what is generated server side (pjax) EZP-26600