The document Kielimerkkaus: tekstin kielen ilmoittaminen merkkausjärjestelmissä (in Finnish) discusses language codes (including ISO 639 codes) in general but especially language markup in markup systems such as HTML and XML, partly in a fairly detailed manner, and emphasizing the accessibility aspect, explaining the related WAI guidelines. Although the principles are simple, there are complicated problems in selecting appropriate markup especially at the phrase and word level, not to mention parts of words. For example, the abbreviation "USA" is clearly of English-language origin; but e.g. in Finnish, it is read using either Finnish letter names or as if it were a Finnish word. Moreover, it is possible that a word in one language takes a suffix from another language. The author recommends that at word level, markup be used to indicate language changes in unproblematic cases only; "if in doubt, leave it out". - As regards documents on language markup in English, the author especially recommends the nice introduction Dan's Web Tips: Languages and more detailed Language Identifiers in the Markup Context. For a deeper discussion, see the W3C working draft Authoring Techniques for XHTML & HTML Internationalization: Specifying the language of content.