Thank you @daniel, that made for a good solution!
As a warning to anyone wanting to do this, you can potentially "brick" your device, so be careful.
This is what I did, after installing xxd and nano:
xxd /dev/mtd3 /tmp/mtd3_xxd_dump.txt
nano /tmp/mtd3_xxd_dump.txt
at 0x88 I replaced "CN" with "DE" and "43 4E" with "44 45".
then I converted it back and wrote it to mtd3:
xxd -r /tmp/mtd3_xxd_dump.txt /tmp/mtd3_xxd_dump_fixed.bin
mtd write /tmp/mtd3_xxd_dump_fixed.bin /dev/mtd3
mtd verify /tmp/mtd3_xxd_dump_fixed.bin /dev/mtd3
After a reboot the UI is working as Global, do note that this produces an empty output after this change:
cat /proc/gl-hw-info/country_code
When set to CN this outputs "CN". I have yet to try if reversing my change will restore this, if it doesn't I suppose I did something wrong above as well.
Exercise caution if you try this, mistyping things can end badly...