It seems we have a misunderstanding here, me explanation was a bit short because I have been through this too many times.
I can not offer any solution in darktable except to avoid any special characters. Maybe someone comes up with a solution that can be scripted in Lua.
The “tagging” part is meant to be done with exiftool (or equivalent tools), writing a tag into the IPTC metadata section inside the file that sets the encoding. This is a harmless process as no other data in the image needs to be changed. I have not used ExiftoolGUI for many many years, so I can not help you there. But the commandline for a folder of jpgs could be as simple as this:
exiftool '-IPTC:CodedCharacterSet=UTF8' *.jpg