FYI: I did translate some strings a while ago. Have a look https://gitlab.com/dadosch/mailman3-german-translation-foo
The reason I didn't made a PR or similar was, that I didn't know where :D
I think you attach those messages in mailman/src/mailman/messages/de/LC_MESSAGES/de.po, and add a README.de at mailman/src/mailman/messages/de/README.de. The README contains information about how to participate in the German translation, such as who is the coordinator (or at least someone willing to read email about it and bring it to our attention), if you're using a translation management site, etc. Then submit the PR at http://gitlab.com/mailman/mailman.
If you are willing to coordinate the German translation, I bet Simon would be willing to cooperate in merging his translation. It would certainly be helpful in getting the process going to have an example of a translation effort for other languages to follow, and it looks like there's a lot of interest in German. So German may be a good place to start.
As I mentioned before, the core developers are not particularly focused on any translation effort. The policy issue about whether to centralize translation management and/or allow use of sites based on non-free software is delicate, as mentioned elsewhere, and there is the issue of coordinating the translation community with release management. Finally, there is the task of translation for Postorius and HyperKitty, which may benefit from a use of Django-specific tools (I'm not familiar with localizing Django applications so that may be completely stupid :-). So this is going to take time to plan.
Please feel free to lobby us, here and on mailman-developers. Contribution of effort (planning or coordination) is not necessary but of course would be welcome. Also, anybody who has participated in translation efforts who can describe a process that results in accurate translations while minimizing core developer and translator pain would be very welcome.