No one under GTK/Gnome uses plain C, they use Glib as a wrapper. Plain ANSI C might be 'unusable' for modern UI needs, but, as I said, just have a look on WebkitGTK4. Glib everywhere, WebkitSettings are a breeze to setup.

Vala it's a toy because Miguel de Icaza went full MS with C# since Ximian. If Vala had more support from Red Hat, Gnome 4 could support Vala as the main language for it. JS? Lua and Luajit wouldb be a better choice for Mutter scripting. If you have a look on how Luakit and Vimb behave, the difference it's almost nil.