I need to switch language using programmation

The theme I am using contents some links in the menu that are only able to be changed by the editor section.
What I need is the name of the keyword to switch that names like:

<?php if ( $lang == ca )?> Categories <?php elseif ( $lang == ES_es )?> Categorias <?php elseif ( $lang == EN_en )?> Categories <?php } ?>

Or something like that. I guess $lang is not the keyword and the name you are using in your plugin is not ES_es, but I need this kind of programmation to translate some other sentences too.

Thanks a lot.

http://ww.wp.xz.cn/plugins/polylang/