There are two ways in which one needs to declare the language of content:

1. to express the language of a specific run of text so that applications that manipulate the text (such as text-to-speech, etc) can correctly understand the text they are currently dealing with,

2. to express the basic language of the document as a whole (this could be used for content negotiation, etc.).

The first type of declaration (what we will call text processing language) must, of necessity, refer to only a single language at a time, though that declaration can be overriden for a labelled fragment of the text, eg. an embedded French word in English text.

The former declaration (what we will call primary language) could involve declaring more than one language, eg. for documents containing parallel texts in multiple languages, but doesn't necessarily mention every language that appears in the document.

For a fuller definition of primary vs. text-processing language see Authoring Techniques for XHTML & HTML Internationalization: Specifying the language of content