Bug 12224: (QA follow-up) Ensure patron language is respected

Bug 12224: (QA follow-up) Ensure patron language is respected

Things have moved on a bit since this code was first written, we now
expect a 'lang' parameter passed into GetPrepparedLetter to allow for
translated notices.

Simple fix, so a quick QA follow-up will suffice :)

Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>