Where to specifiy the language of Document in HTML 5 when my page has two languages?
You can only define one language as the HTML's primary language. There can be only one language (no multiple values) specified for the <html> tag's lang attribute. But you can define a different language for some parts of the page.
For example:
<html lang="en">
...
<p lang="de">Ja, Deutsch.</p>
See the HTML5 spec for further info.