Hi all!
I upgraded to 1.5.8 from 1.5.7 last week. Aside from a few things here and there, everything went fine.
I noticed that in this release, the language files changed to lang.FILE.php files instead of FILE.php files.

While testing orders, I noticed that my site wouldn't respond when clicking the continue button on the checkout_payment page. Each time I clicked the button, nothing would happen visually, but the system would generate an error file with the following:

Code:
Request URI: /ajax.php?act=ajaxPayment&method=prepareConfirmation, IP address: XXX.XXX.XXX.XXX
#1 require_once() called at [/includes/classes/ajax/zcAjaxPayment.php:177]
#2 zcAjaxPayment->prepareConfirmation() called at [/ajax.php:85]
--> PHP Warning: require_once(/includes/languages/english.php): failed to open stream: No such file or directory in /includes/classes/ajax/zcAjaxPayment.php on line 177.

PHP Fatal error: require_once(): Failed opening required '/includes/languages/english.php' (include_path='.:/usr/share/php') in /includes/classes/ajax/zcAjaxPayment.php on line 177
As far as I can remember, I don't have any custom code or edits on the payment/checkout modules.
If I re-add my previous english.php file, it throws more errors because that file links to other language files that were changed in this release.

Checking out using PayPal works fine, it's just when I attempt to process a credit card directly on the site that I get this error.
I figured before I started hacking away, I'd ask and see if anyone else has had this issue.

Thanks!