Joost: I don't think it's necessarily a ranking factor. I do think that if you do all the technical stuff around multilingual SEO well and you have a page ranking well in English and you have a page in Spanish then the fact that you have an English page that hreflangs correctly to that Spanish page might be helping that Spanish page. In that case, it's not the fact that you have multiple languages, but it's the fact that you have multiple places in which you can rank and gather links and whatnot. Having a translated version of your website can be beneficial.