This guideline requires specification of language for html elements that use a different language than the specified language of the web page. When a change in language occurs, such as quotes or translations, assistive technologies and user agents can provide more accurate renderings when they're given a heads up.
The Official WCAG Guidelines recommend specifying any language changes by simply using lang attributes.
<p>In order to practice his spanish, Waldo responded, "<span lang="es">Lo siento mi amigo Jeff.</span>"</p>
Foreign words that become common in other languages do not need lang attributes (ex: rendezvous, burrito).