Bug 33237: Move and simplify language selection logic

Bug 33237: Move and simplify language selection logic

This patch moves the language selection into GetPreparedLetter, drops
the interface check step and ensures the default template always uses
the default include language as per the OPACLanguages settings.

Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Katrin Fischer <katrin.fischer@bsz-bw.de>
(cherry picked from commit c50bb140a872e67e07426fc3fc20bceb88c7d1e3)
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>