To change the iFrame version “like” button’s language, it is sufficient to add the locale to the UR, e.g. “?locale=de_DE”.
With the XFBML version, language settings are made globally via the respective Java script. That is, instead of
<script type="text/javascript" src="http://connect.facebook.net/en_US/all.js"></script>
just
<script type="text/javascript" src="http://connect.facebook.net/de_DE/all.js#appId=132298656846430&xfbml=1"></script>
This post is also available in German
