I\'m using DalekJS with PhantomJS to test a web application. I have important assertions which depend on the browser language. And these fail if the language is not English.
Let me answer my own question:
The problem is solved when you use following code in your index.html:
<script>
localStorage.lang = 'en';
</script>
So the site will be displayed in english no matter which browser is used.
This question has been further discussed here: https://groups.google.com/forum/#!topic/dalekjs/HHt6eoZRQc4