For me it is China after I noticed that a lot of them uses some kind of strange client that always shows that they have 0 progress to get better priority from other pears. Later after I started to look into it I noticed that they do not even seed.
It's not automatic, but you can technically block countries. From the Connections tab in Options, there is an IP Filter option for data files. I believe the format is X.X.X.X-Y.Y.Y.Y
Country IP assignments are handled by ARIN in North America and RIPE NCC for Europe. Those are the two main ones, but LATAM, Africa, and APAC territories have their own respective groups as well. So, every main German IP block is known and searchable via RIPE. You would have to format your lists using that info.
It may not be super effective as IPs can somewhat float, but that would be the method.
It's posible to use a file too. The is an option just above that filter field. I think it's more convenient, because there might be a lot of small ranges. The file just have to be formated in specific way.
I remember I even found a website that allowed to export ranges for specific country in a file and I made a small Python script to format that file.
Yeah, I was referring to the field that takes in dat files. Honestly, it sounds like the OP is more interested in just outright blocking countries completely with Qbittorrent just one application abidding by that. At that point, OP should take the IP ranges and script them into iptables statements. I've never created 1000+ iptables configurations though so I don't know what kind of performance hit that creates if any.