Simple Linux keyboard switching: Carillon

As part of my work on my custom Linux distribution, I needed an application that: had a graphical user interface, listed a set of keyboard layouts, applied them when changed, used low-level X components only. A very simple set of requirements. And yet, I couldn’t find anything. Most of them are tied to a specific desktop environment, like GNOME or KDE. Or they use their own complex components, like iBus.