Upgrader don't know which language are using site. Site_langauges= "en,es" means you site is available in english (en) and spanish(es), and widget are written in english (en). Languages will be detected in url so: /es/foo and /en/foo is processed as foo with language = es or en.
A site can work without this values. Then Aiki assumes english and will not detect language (backward compatibility).
Setting the language is responsibility of web developer who will use the admin.