As long as you have no intention of selling your own carts of the German version, then you have my blessing. There could potentially be an "official" release one day, but as of now the German version didn't make enough money to warrant a full physical edition.
I'm continuing to promote the series as much as possible, especially with the sequel in development. A lot of people are still only now discovering the first game, so if demand increases for a physical German version I may consider it (after all, it's the version that actually has a full Super Game Boy border).
If you do make your own cartridge out of pure fan passion, I'd definitely be interested to see photos! Thanks for asking.