How to set the document language based on the contents of a text variable?

In my Indesign documents, I use a text variable in which I type the abbreviation of the document language according ISO639-1 (e.g. EN, DE, ES, ...). I'd like to have a JavaScript that checks this text variable every time the document is opened and then sets the Language property of my Normal paragraph style, but I don't have any experience with JavaScript.

Can someone help me please?

PS: I use InDesign CS5.5.