How to set default values for edit_post_link() in my theme?

You can write your own hook for edit_post_link filter:

add_filter( 'edit_post_link', 'wpse8170_edit_post_link' );
function wpse8170_edit_post_link( $link ) {
    $matches = array();
    if ( !preg_match( '/\>.*?\<\/a\>/i', $link, $matches ) ) {
        return $link;
    }
    return str_replace( $matches[0], '>MY NEW EDIT LINK TEXT HERE</a>', $link );
}