A multi-language site offers several versions of the same content, one version for each of the languages in the site. To make search engines aware of this structure, each page much include additional information that allows the search engine to identify the alternate versions of the page. Otherwise, those pages could be treated as duplicates, or not be included in the index.
Google gives some guidelines on how to include these metadata. Those guidelines are briefly summarized in this post.
1. Use a distinct url for each language
If the main domain of a web site is www.example.com, the french version could use the subdomain fr.example.com, the italian version could use it.example.com, and so on.
Alternatively, different languages could use a structure of subdirectories of a single domain: www.example.com/fr, www.example.com/it, etc.
Besides, if the URI of a given page is made up of keywords, it is advisable to also translate the uri itself. For instance, if the URL of a page written in english is http://www.example.com/how-to-setup-a-multi-language-website-for-google, the translation into spanish could use the url http://www.example.com/es/como-preparar-un-sitio-multi-idioma-para-google.
2. Use the hreflang meta-tag
Inside the <head> section of the HTML code of the page, there should be a hreflang meta-tag for each of the versions of the page.
For instance, in the page http://www.example.com/how-to-setup-a-website:
<link rel="alternate" hreflang="es" href="http://www.example.com/como-preparar-un-sitio" />
<link rel="alternate" hreflang="en" href="http://en.example.com/how-to-setup-a-website" />