Bug 8347 - Koha forces UNIMARC 100 field code language to 'fre'

Bug 8347 - Koha forces UNIMARC 100 field code language to 'fre'

Added a new system preference to set the UNIMARC field 100 default language.
The default value for that system preference is 'fre'.
Changed Biblio.pm to use the system preference:

- if the language is bad filled in the preferences it uses 'fre' as default value
- only replaces the language when the field 100 is empty
- if the language is filled with the plugin only replaces the positions 25-28 to 'y50'

Signed-off-by: Rolando Isodoro <rolando.isidoro@gmail.com>
Signed-off-by: Katrin Fischer <Katrin.Fischer.83@web.de>
Signed-off-by: Jared Camins-Esakov <jcamins@cpbibliography.com>