How can I replace "English" with "EN"?
I use this, it's a little bit more dynamic then if you would hardcode the language code yourself. Hope it helps. This example works with the language switcher block.

function theme_preprocess_links__language_block(&amp;$variables) {
    foreach(array_keys($variables['links']) as &amp;$code){
        $variables['links'][$code]['link']['#title'] = $code;
    }
}