How to default a content type's Language field to the authenticated user's language?
actually, i would create a custom module (i hope, you know how to code a bit)

the code should be something like the following:

    &lt;?php
    function YOUR_CUSTOM_MODULE_node_presave($node){
        global $user; // gets the current user account
        $node-&gt;language=$user-&gt;language;
    }