Yeah? You at the very least forgot to obtain the source code first somehow. What about build-dependencies, because ./mach bootstrap does not fully handle that?

Now please tell me how to do a Windows release-build with all release features enabled (except for official branding), aka. a ton of configure switches, and also please do it for my language using the official de locale, because neither the source tar.bz2 nor the hg you'd normally clone contains that. I'm starting from scratch of course. And suddenly it is less easy and trivial..

As the link mentions, you don't need to build from source. Binary builds are provided that do not have this restriction, for those that want that.