Find out when keyboard layout is changed
There is a way ...
First you need to register your application to capture foreground window changes:
Use SetWinEventHook with EVENT_SYSTEM_FOREGROUND (and WINEVENT_OUTOFCONTEXT as it's .NET) for that.
If that happens: Use your GetKeyboardLayout solution for getting the current layout of that window.
Then use a local Windows Hook (you're probably using it low-level-globally for key captures) with WH_CALLWNDPROC and the thread of the new foreground window.
Listen to WM_INPUTLANGCHANGE messages to that window to receive changes to the layout.
(You may want to unhook/rehook after another foreground change)
It looks like the keyboard layout is stored here: HKEY_CURRENT_USER\Keyboard Layout\Preload
When I changed keyboard languages, the order of settings there changed.
So you could possibly monitor the registry entry. Here's one way: