The bitnami AWS installation creates the database before you have a chance to set the defaults. So, to answer my question, you need to:
rm /home/bitnami/apps/shopware/htdocs/install.lock
mysql -u root -p
drop database bitnami_shopware;
create database bitnami_shopware;
quit;
and rerun the installation which will set up the database again and THEN ask you what the default language and currency is.
All a bit opaque what…