I tried:

env KOHA_CONF=/etc/koha/koha-conf.xml env PERL5LIB=/usr/share/koha/lib perl
translate install de-De

but got:

readdir () attempted on invalid dirhandle $fh at LangInstaller.pm line 65.
closedir () attempted on invalid dirhandle $fh at LangInstaller.pm line 66.
Unsupported language: de-DE

many thanks