How can I set the language of a page?

Hello,

If I have 10 sections within the Web site, each one for a different language, how can I make the pages be indexed under each language (as opposed to the whole site being in one language)?

I don’t know how to go about this, but does that language end up in this spot:
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd”>
In other words, the "//EN” bit?

Or can an WordPress entire site ONLY have one language, for example (attached).

Or ... does each language be on its own subdomain with its own install of WordPress (that would be messy).

Thank you,
Michael Charters