[CentOS] rpmbuild: definition of the %prep script

>Actually I'm trying to figure out where these lines originate from

I see

>Executing(%prep): /bin/sh -e /var/tmp/rpm-tmp.86989
>+ umask 022
>+ cd /home/user/rpmbuild/BUILD
>+ LANG=C
>+ export LANG
>
>The setting of LANG messes up my build, which relies on Utf8.

So export this in the build environments shell, afaik if its previously
it will not redefine it.

export LANG=en_US.UTF-8
rpmbuild <...>

Try that out.