I understand that, and the answer lies in a change of the setup script that was made months ago. That was supposed to be a modernization, which is necessary for modern Debian versions, I think, but the install part has a fatal error, that one can only find by running the install and typing gramps, or starting Gramps from the menu.
And since the installer is destructive, in the sense that it overwrites your existing installation, and there is no remove either, it is quite nasty to test, also because most things look normal. When I run it, the locales seem to be there, but they don’t work in LMDE 6, based on Debian bookworm, and they don’t work on ubuntu either, which is based on an older Debian version.
And when this happens, running a 5.1 installer, from source, or from a .deb file, doesn’t work either, so you need timeshift to roll back your whole system. That works here most of the times, but I prefer to avoid that in my production environment, because it can be risky.
We have a thread with a lot of technical details here, started by Jean Michault:
And I hope that we can find ways to test source distributions including build and install on Arch, Debian, Fedora, and ubuntu, before releasing. And I know that that might not always be easy.