Primary language is typically associated with the intended audience of the content.

For example, a Japanese phrase book for English tourists may contain a lot of Japanese text, but the primary language of the book is English.

There is some content on the Web that provides parallel content in more than one language in the same document. This is rare, given the ease of linking between different language versions on the Web, but still occurs from time to time. In this case the document in question has multiple primary languages.

You may also come across pages that have been localized quickly or at low cost, and where the essential content has been translated but the navigation and other information on the page is still in the original language. In these cases, even if the translated material is only a small part of the document, it could be argued that the primary language is the translated language, since that is the language of the intended audience.