If you wish, you can force a certain language for your live chat visitors - default language is English (en).
Default ISL Pronto script line looks like this:
<script type="text/javascript" src="http://pronto.isl.example.com/live/islpronto/public/chat.js?d=default"></script>
If you wish to force a certain language, just add the lang=<language> parameter (where language is a two-letter language code) to the script line, like this:
<script type="text/javascript" src="http://pronto.isl.example.com/live/islpronto/public/chat.js?d=default&lang=de"></script>
Appending &lang=de like in this example will force language to Deutsch (de).