Install an additional language to an existing store by executing following steps (replace Store and fr by your choice):

STORE=Store
LANGUAGE=fr
. /etc/default/epages6
cd $EPAGES_CARTRIDGES/DE_EPAGES/Installer/Scripts
$PERL updateForLP.pl -storename $STORE -langpack $LANGUAGE
$PERL importDBStrings.pl -storename $STORE -lang $LANGUAGE

Add new languages to stores with different store types by executing those commands for each store and language.