GIMP cannot be run if its installation path contains any special characters

GIMP version: 2.10.18 (and 2.10.14, but NOT with 2.8.22)

Operating System: Windows 10 Pro 64-bit, version 1019, build 18363.836

Package: GIMPPortable_2.10.18-2.paf.exe (GIMPPortable_2.10.14-1.paf.exe or GIMPPortable_2.8.22.paf.exe, respectively) installer from the PortableApps.com project on sourceforge.net

Description of the bug

GIMP Portable does not start at all if it is installed, i.e. extracted, in a folder whose name contains special characters like the German ä, ö, ü, or ß (details see below).

Reproduction

The bug is always reproducible with GIMP Portable 2.10.18 or 2.10.14 (supposedly, but not tested, all 2.10.* versions are affected), but never with version 2.8.22.

Reproduction steps:

1. Execute the GIMPortable_2.10.*.paf.exe and install GIMP in a folder containing at least one of the characters ä, ö, ü, or ß.
2. After the installation has been completed try to execute either GIMPPortable.exe, App\gimp\bin\gimp-2.10.exe, or App\gimp\32\bin\gimp-2.10.exe.
3. Nothing happens except a window flashing up for a few milliseconds (no chance to see what it is and there is no error message).
4. If instead one tries to run App\gimp\bin\gimp-console-2.10.exe or App\gimp\32\bin\gimp-console-2.10.exe within cmd.exe, this will yield error messages similar to the following one:

../babl-0.1.74/babl/babl-internal.h:214 babl_log()
        WARNING: the babl installation seems broken, no extensions found in queried
BABL_PATH (G:\GIMPPortableä\App\gimp\lib\babl-0.1) this means no SIMD/instructions/special case fast paths and
only slow reference conversions are available, applications might still
run but software relying on babl for conversions will be slow

../babl-0.1.74/babl/babl-internal.h:222 babl_fatal()
        babl_format("CIE Lab double"): not found
Der Befehl "grep" ist entweder falsch geschrieben oder
konnte nicht gefunden werden.

5. Looking at the path in the third line, I'm guessing that the issue is somehow caused by an encoding problem. Probably, ASCII is used for handling file names instead of some kind of Unicode!? (The last two lines say: The command "grep" is either misspelled or could not be found.) The GUI executables from step 2 produce very similar error messages if they are executed within cmd.exe with the parameters --verbose --console-messages.
6. If the folder is renamed such that the initially chosen special characters are removed, all executables can be run without any problems.

Expected result: All GIMP executables should run without any problems, no matter what the installation directory is called…

Actual result: … but if its name contains special characters, like ä, ö, ü, or ß, it's impossible to successfully execute the program(s).