Comment from the i18n review of:
Location in reviewed document:
3.1.1 Example 1
"A Web unit produced in Germany includes content in both German and English, but most of the content is in German. The primary natural language is identified as German (de)."
If the primary language is expressed using HTTP or meta tags, it is possible that both languages should be identified if this is a document aimed at a bilingual audience. If the primary language is to be expressed in the html element tag, only one language can be chosen. This example is too vague. This goes back to the question of what WCAG means by 'primary language'.
We have clarified our use of primary language to be the default human language of the Web page, and we changed SC 3.1.1 to read "The default human language of each Web page within the content can be programmatically determined." We included a reference to Internationalization Best Practices: Specifying Language in XHTML & HTML Content, and added a discussion of multilingual documents to the Intent section. We added "default" to the example to make it clearer why this satisfies the SC.
Currently assistive technologies do not support specifying languages in HTTP headers or meta tags, so those techniques are not considered sufficient at this time.
HTTP headers and meta tag marking of languages can identify multiple languages, as you point out. Specifying multiple languages in the http header or in meta-data would not specify a default text processing language, so such usage would not satisfy this success criterion. This would be discussed when those techniques are written.