How does Google handle a page containing multiple languages?

Matt's answer:

Matt Cutts: Eddy asks an interesting question from Milan, Italy. Eddie wants to know how would Google consider and rank a site that uses meta data and URL’s and a language, for example, Italian, and has the H1 of the pages in another language, for example, English, considered more appealing for users. So the title might be “Pallacanestro” and the H1 might say “basket”, for example. Well the short answer is that we try to handle things as best we can. So even if you mix different languages on a page, we’ll try to interpret that correctly, identify the language of the page, and then return it when we think it’s most appropriate, whether it be for Italian language searches or for English searches. But I would say as a best practice you normally want to make sure that all of the stuff on the page is in a single language if you can. Now, for example, if you’re quoting someone who’s talking in a different language, it might not be possible to make sure that it’s always the same language. But to the degree that you can, I would recommend having pages all in a single language. If that means that you end up having one Italian page and one English page, that’s completely fine because it can rank for both Italian and regular American or British users. But if you can, I would avoid sort of interleaving those languages because we try to say what is the most dominant or the-the words that have – which language occur the most in terms of language on the page, and if we have trouble doing that then it might not rank where you would like it to rank for your users. So you certainly can mix the languages. We’ll try to do the best that we can, but if you are at all able to, I would say try to separate different languages onto different pages.

by Matt Cutts - Google's Head of Search Quality Team

Original video: