[atrezonator]

Originally Posted by kovidgoyal

set the environment variable
CALIBRE_OVERRIDE_LANG=ru
though I dont know how complete the russian translation is.

[Unit]
Description=calibre content server
After=network.target

[Service]
Type=simple
User=calibre
Group=calibre
CALIBRE_OVERRIDE_LANG=ru

ExecStart=/opt/calibre/calibre-server \
--access-log=/var/log/calibre-server-access.log \
--enable-auth \
--log=/var/log/calibre-server.log \
--port=8008 \
--url-prefix=/calibre \
--userdb=/var/lib/nethserver/calibre/users.sqlite \
"/var/lib/nethserver/calibre/library"

[Install]
WantedBy=multi-user.target

Like this ?