It's mostly irrelevant. You can rank with just com/org/net PBNs that have solely english content, as long as the anchors are properly adjusted. Having said that, rationality dictates that some PBNs with german content wouldn't hurt. It really depends on the overall goals you want to achieve with the site, your risk aversion, longevity etc.
But you do not need .de PBNs - further, I wouldn't really recommend using .de domains for PBNs anyway, because you can't hide your privacy well, which in turn leaves a decent footprint.
It doesn't matter what thy name or extension of the domain is.
The vital thing is to host it on a .de server and IP.
You can host the website on the Moon if you want to - as long as it can be indexed by the search engines, it doesn't matter where the physical website files are located. It only matters for website speed (visitors having a lower latency will have higher bandwidth speeds and vice versa), but since everyone can use CloudFare nowadays or any other CDN, that's irrelevant as well.
@OP: You will rank equally well by using top TLD PBN-s as opposed to using only .de ones, if not better. As someone said above, a lot of country-specific TLD-s cannot be used with WHOIS privacy, so you will be leaving a major footprint there (if all the data is the same). If I were you, I would keep the ratio around 80:20, so for every 10 PBN links, only 2 are .de domains. I've used the same ratio in 3 foreign niches (countries in Europe) so far without any problems.
Last but not least, depending on the TLD and country, such expired domains will be way more expensive than normal ones because of scarcity. If you scale up to hundreds of links, the costs will add up quickly with almost no benefit at all.
You can perfectly rank this way, true, but it's not irrelevant for one reason: If you are using an english content PBN and german anchors, Google will think the linked money site is english language, although the anchors are german and the site is 100% german and show german users in Google.de a "translate this site" in the SERPs that totally crushes CTR.It's mostly irrelevant. You can rank with just com/org/net PBNs that have solely english content, as long as the anchors are properly adjusted.
It's mostly irrelevant. You can rank with just com/org/net PBNs that have solely english content, as long as the anchors are properly adjusted. Having said that, rationality dictates that some PBNs with german content wouldn't hurt. It really depends on the overall goals you want to achieve with the site, your risk aversion, longevity etc.