Use <acronym> and <abbr> to expand all acronyms and abbreviations using the title attribute:
Some browsers style these using dotted underlines, etc.
Acronym v abbr? There is much debate and confusion about what exactly constitutes an acronym versus an abbreviation. Also there is different support across browsers, and changing support in new standards (i.e. dropping acronym). Best advice is to use consistently and to always expand the abbreviation somewhere in the main body of the text.
Examples
<html lang="en">
...
<p>How do you say hello in French?</p>
<p>You say
<span lang="fr">Bonjour</span></p>
<p>And in German?</p>
<p lang="de">Guten tag </p>