OK here's what you have to do for newer versions of OPC that have the 1.5.8 style language files (the ones whose names start with lang.*)
Note that this is for Zen Cart 1.5.7 only - the 1.5.8 plugin will require significant rework.

at the top of admin/includes/functions/extra_functions/preview_email.php add:

Code:
$opc_funcs = DIR_FS_CATALOG . DIR_WS_FUNCTIONS .  '/extra_functions/one_page_checkout_functions.php';
if (file_exists($opc_funcs)) {
   require $opc_funcs;
}
in includes/functions/extra_functions/one_page_checkout_functions.php line 128 change

Code:
    $base_language_dir = DIR_WS_LANGUAGES . '/' . $_SESSION['language'] . '/' . $legacy_dir;
to

Code:
if (!defined('IS_ADMIN_FLAG') || IS_ADMIN_FLAG !== true) { 
       $base_language_dir = DIR_WS_LANGUAGES . '/' . $_SESSION['language'] . '/' . $legacy_dir;
} else {
    $base_language_dir = DIR_FS_CATALOG_LANGUAGES . '/' . $_SESSION['language'] . '/' . $legacy_dir;
}