The Nextcloud AIO container itself doesn’t have access to the backup directory, but it has access to the docker socket (/var/run/docker.sock). Having access to the docker socket means it can perform any docker operation on the host system, in this case starting a separate backup container with the backup directory mounted.

I’m aware of LWT and LUTE with the same concept. Neither comes with predefined languages or texts, so they should work for any language as long as you have some texts you want to read.