And what percentage of "all programming languages" is that?
No, there really aren't that many. Even fewer that are "professionally" usable. (I am not committing to Go's toolkit joining that latter set as I've not yet seen or used it.)
Python has a default cross-platform UI library?
Yes, Tkinter, a Tk wrapper.
You mean apart from Tcl, Python, Java, Factor, just about every Smalltalk apart from GNU Smalltalk, Racket, possibly Ruby…