I proceeded to rename and then uploaded the folders. both language folders are now "en-GB" but still the module doesn't display the correct text. I've contacted the developer of the module as well and hope to get a response. Can anything else cause this problem? I assume this is the code that is supposed to display to redirect to the correct wording. It's in the file called firstdatae4api.php.bak in admin/language/en-GB/

/**
* Opencart FirstDatae4api Payment Module - Admin Language
*
* NOTICE OF LICENSE
*
* This source file is subject to the Open Software License (OSL 3.0)
* that is bundled with this package in the file LICENSE.txt.
* It is also available through the world-wide-web at this URL:
* http://opensource.org/licenses/osl-3.0.php
* If you did not receive a copy of the license and are unable to
* obtain it through the world-wide-web, please send an email
* to [email protected] so we can send you a copy immediately.
*
* @category Opencart
* @package Payment
* @copyright Copyright (c) 2010 Schogini Systems (http://www.schogini.in)
* @license http://opensource.org/licenses/osl-3.0.php Open Software License (OSL 3.0)
* @author Gayatri S Ajith
*/
// Heading
$_['heading_title'] = 'First Data Global Gateway e4 - Webservices API';

// Text
$_['text_payment'] = 'Payment';
$_['text_success'] = 'Success: You have modified your First Data Global Gateway e4 Webservices API payment method details!';
$_['text_firstdatae4api'] = '';
$_['text_test'] = 'Test';
$_['text_live'] = 'Live';
$_['text_authorization'] = 'Authorize Only';
$_['text_capture'] = 'Authorize & Capture';
$_['text_edit'] = 'Edit First Data Global Gateway e4 Webservices API';

$_['text_mastercard'] = 'Mastercard';
$_['text_visa'] = 'Visa';
$_['text_diners'] = 'Diners';
$_['text_amex'] = 'American Express';
$_['text_maestro'] = 'Maestro';

$_['text_payment_info'] = 'Payment information';
$_['text_capture_status'] = 'Payment captured';
$_['text_void_status'] = 'Payment voided';
$_['text_refund_status'] = 'Payment refunded';
$_['text_order_ref'] = 'Order ref';
$_['text_order_total'] = 'Total authorised';
$_['text_total_captured'] = 'Total captured';
$_['text_transactions'] = 'Transactions';
$_['text_column_amount'] = 'Amount';
$_['text_column_type'] = 'Type';
$_['text_column_date_added'] = 'Created';
$_['text_confirm_void'] = 'Are you sure you want to void the payment?';
$_['text_confirm_capture'] = 'Are you sure you want to capture the payment?';
$_['text_confirm_refund'] = 'Are you sure you want to refund the payment?';
$_['text_capture_ok'] = 'Capture was successful';
$_['text_capture_ok_order'] = 'Capture was successful, order status updated to success - settled';
$_['text_refund_ok'] = 'Refund was successful';
$_['text_refund_ok_order'] = 'Refund was successful, order status updated to refunded';
$_['text_void_ok'] = 'Void was successful, order status updated to voided';


// Help
$_['help_total'] = 'The checkout total the order must reach before this payment method becomes active';
$_['help_cards_accepted'] = 'Ensure that your merchant account supports American Express and Maestro before selecting them';
$_['help_settle'] = 'Authorize means the amount is blocked in the customers card but, not yet credited to your account. You need to capture it for it to reach your account.';
$_['help_debug'] = 'Enabling debug will write sensitive data to a log file. You should always disable unless instructed otherwise';

// Tab
$_['tab_account'] = 'API info';
$_['tab_payment'] = 'Payment settings';

// Entry
$_['entry_login'] = 'Store ID:';
$_['entry_username'] = 'Username:';
$_['entry_gatewayid'] = 'Gateway ID:';
$_['entry_gatewaypasswd'] = 'Gateway Password:';

$_['entry_mode'] = 'Transaction Mode:';
$_['entry_method'] = 'Transaction Method:';
$_['entry_total'] = 'Total:';
$_['entry_cards_accepted'] = 'Card types accepted:';
$_['entry_order_status_capture'] = 'Captured Order Status:';
$_['entry_order_status_authorize'] = 'Authorize Only Order Status:';
$_['entry_order_status_declined'] = 'Failed Order Status:';
$_['entry_geo_zone'] = 'Geo Zone:';
$_['entry_status'] = 'Status:';
$_['entry_sort_order'] = 'Sort Order:';
$_['entry_debug'] = 'Debug logging';

// Error
$_['error_permission'] = 'Warning: You do not have permission to modify details of the payment method FirstData!';
$_['error_login'] = 'Store ID Required!';

$_['error_username'] = 'Username Required!';
$_['error_gatewayid'] = 'Gateway ID Required!';
$_['error_gatewaypasswd'] = 'Gateway Password Required!';

// Button
$_['button_capture'] = 'Capture';
$_['button_refund'] = 'Refund';
$_['button_void'] = 'Void';
?>

You should rename them before uploading then upload. Otherwise you should copy their folders.

I found two folders named english in the catalog/language and admin/language folder. I have renamed to en-GB but problem remains.

Sorry, I pressed submit several time because nothing seemed to be changing. I've attached a screenshot as well.

That is because of the language folders. You should rename any "english" folder to "en-GB".

Can't say anything about specific themes but here you can find more info about compatibility: http://arastta.org/docs/developers/opencart-compatibility