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From Elxis Official Documentation

To create a parameter for your module, bot, component, or template XML file that will list all installed languages automatically open your extension's XML file and create a parameter of type folderlist (requires Elxis 2009.1+). As value for the directory attribute use the relative path to the elxis language folder.

<param name="lang" type="folderlist" directory="/language" default="english" label="Language" description="Select language" />

When rendering parameters Elxis will generate for you a list of all installed languages as a drop down selection list. You can see the html output in the screen shot bellow.