Copy directory with plugin to plugins directory of your Omeka installation.
Go to /admin/plugins in browser and click "Install" button in "Language selector" row.
Go to /admin/plugins in browser and click "Configure" button in "Language selector" row.
There will be a form with only one field. Data in this field will be used to generate <select> tag.
Each row is one option in <select> tag.
Syntax:
locale;label
where locale should be one from listed at this page and label will be used for particular option.
Insert the following code in any place in your theme's view file you want to place the widget:
<?php fire_plugin_hook('language_selector', array('view' => $this)); ?>