CRM-11458 Realex payment gateway - strings not translated, expiry date errors

I'm using the Realex payment gateway for processing donations. As I'm using it on a multilingual site, I changed the date format drop-down to be 'm' rather than 'M' (i.e. 01, 02, etc rather than Jan, Feb, etc) because it wasn't possible to translate the month names. However, once I did that the Realex payment stopped working as it expected the date format to be M. It was using the $params['credit_card_exp_date'] value directly, rather than the $params['year'] and $params['month'] variables. I changed the code to use these variables instead and now the transactions are working again.

Further to that, the error messages returned to the user on failed transactions (e.g. if the card was declined) weren't present in the po files for translation. So this patch also adds in ts() calls around all untranslated strings in this file.

Attached patch fixes both issues.