Translate ssowat manually

It looks like is down, so I translated en.json locally and copied my translation to directory /usr/share/ssowat/portal/locales on my Yunohost VPS. Based on I added

"default_language": "cs"

to /etc/ssowat/conf.json.persistent. This led to an error on access to /yunohost/sso/ relative URL. So I tried to replace en.json file with my translation and changed /etc/ssowat/conf.json.persistent back. Which didn’t help, everything is still in English. What can I do to make it work?