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Best language: I don't think anybody can answer this satisfactorily. Just pick what you are best and most comfortable at. However, I would recommend to pick an object-oriented language for GUI development (particularly if it is going to be a big project).
Best framework: I would highly recommend you take a look at https://www.qt.io/. It's probably the most ...
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So as performance was mentioned, C++ is the language of choice then. One of the GUI APIs was already mentioned Qt, which seems to be a wery popular GUI API. I personaly not a fan of Qt so will point to the alternative, which is wxWidgets. wxWidgets is cross-platform and open sorce, but other then Qt it´s 100% standard C++ without custom keywords.
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You might like https://www.lazarus-ide.org/, which is free, and lets you build nice GUIs by point-and-click. Programming is in Pascal.
Alternatively, there is TKinter, also free, which is more-or-less the default GUI library for the Python language. Although I find it easier to do many manipulations of various kinds of objects in Python I often find that I ...
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In addition / alternative of Qt, you could also try GTK (used in Gnome) or FLTK or FOX.
If you develop a gaming application, try SFML. Perhaps you want OpenGL support?
Do you care abound sound ? 2D or 3D drawings ?
GTK is usable from C code, from C++ code (with GTKmm), from Ocaml code (with lablgtk), from Python code, etc....
All of Qt and GTK and FLTK and ...
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