Bug 17762: Send lang to GetPreparedLetter

Bug 17762: Send lang to GetPreparedLetter

This patch set the lang parameter when C4::Letters::GetPreparedLetter is
called to generate the notice.
Note that we do not need to pass it if want_librarian is set.

TODO: I do not know what to do with TransferSlip

Sponsored-by: Orex Digital
Signed-off-by: Hugo Agud <hagud@orex.es>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>