Ertan Küçükoglu offered to try to review and test this, so here's a rebase.
Some notes:
* it turned out that the Turkish i/I test problem I mentioned earlier in this thread[1] was just always broken on Windows, we just didn't ever test with UTF-8 before Meson took over; it's skipped now, see commit cff4e5a3[2]
* it seems that you can't actually put encodings like .1252 on the end (.UTF-8 must be a special case); I don't know if we should look into a better UTF-8 mode for modern Windows, but that'd be a separate project
* this patch only benefits people who run initdb.exe without explicitly specifying a locale; probably a good number of real systems in the wild actually use EDB's graphical installer which initialises a cluster and has its own way of choosing the locale, as discussed in Ertan's thread[3]