Identify default language of document:

<html lang=“en”>

Identify changes in language:

<p lang=“fr”>Après la pluie le beau temps.</p>

Identifying language in PDF (using Adobe Acrobat Pro)

Document Properties dialog in Adobe Acrobat, includes a Language select field under the Advanced tab

More on Language

Experiments with Language and Accessibility - Terrill Thompson's blog post includes links to Lang Attribute Tests, with sound recordings of screen readers, plus The Accessible Twitter Translation and Lang Experiment (TATTLE).