No big secret. For some personal, i needed a small GUI application. It looks reasonable to just write one for your needs.
Electron looks overkill. Also C++ with QT5. I don't need multiple inherintence and abstract classes in my case (Sorry Bjarne Stroustrup)
Then these things came into my mind.
ports-mgmt/psearch and/or ports-mgmt/portfind help to find such. In this particular case of the single character D language, of course it's neccessary to know the two compilers are called dmd & ldc, resp.: psearch -c devel -c lang -s 'dmd|ldc' gives 4 false positives & 2 helpful answers.
D is very appealing as a language to implement all kind of tasks from low level kernel stuff to GUI apps. Looks that it's levering/integrating some modern techniques & methods better than C/C++, where these are add-ons. Maybe Rust is even better?
Too bad Binding to the "huge" Qt is not for the faint of hart. Not only pascal but also nim,crystal, d-lang and many other languages struggle with it.
The absence of Qt-binding is not an indication of a toy language but rather the absence of a huge userbase on which i.e. python can rely.
It's a bit strange Qt-people make magnificent widgets but are unlucky "language-binding-unfriendly".
If I recall, the proprietary product (https://www.embarcadero.com/products/delphi) that fpc is modelled after had an original UI system (called Visual Component Library) but now I believe that is actually layered ontop of Qt.
I know the forefather, Borland Kylix (C++Builder for Linux) provided CLX which was an implementation of VCL as a thin wrapper over an old version of Qt.
So it is possible... Just a big project I suppose.
Edit: I used to love Borland C++. Version 6 (and Kylix) was the pinnacle but after that the IDE got horribly bloated. I think CodeGear bought it and did some terrible things before Embarcadero picked up the pieces and added more bloat of its own.