Gengo and templates

I’m testing the Gengo multilingual plugin + WordPress 2.2 because I need to put different text according langage in a template (I have previously tested Polyglot but it doesn’t work in templates).

I’d like to know how to modify templates (single.php) to tell to Gengo to display a french version of a code / english version of a code :

<?php echo c2c_get_custom('doc_primary_source', '<font color="red">Primary source: </font>', '', 'none'); ?><br/>

and, in French :

<?php echo c2c_get_custom('doc_primary_source', '<font color="red">Source: </font>', '', 'none'); ?><br/>

Is there some tag/function like with Polyglot ([lang_xx][/lang_xx]) with Gengo?