I couldn’t find how to change the website language, so here is my current solution.
1) Create a plugin (change ’nl’ to the correct language):
<?php
$modx->setOption('cultureKey', 'nl');
?>
2) Activate the ’onInitCulture’ event on the plugin’s ’System Events’ tab
3) Save the plugin
After this your snippets should use the language of this plugin. Offcourse you can change this plugin to set the language based on the current context (or some other condition).