I have four keyboards and I have defined Ctrl+1, Ctrl+2 and Ctrl+3 ... hot-keys for them. The problem is that either all of the hot-keys or the first one is removed regularly. For example every time the computer goes to sleep (or even locked) all of them are removed.
I even tried other hot-keys (Ctrl+Shift+1, ... Ctrl+Shift+4) or (Alt+Shift+1, ... Alt+Shift+4) and the problem still exist.
If I assign a transition key (Left Shift+Alt) it will work but that's not that useful when you have more than 2 keyboards and you want to regularly change the keyboard.
It appears that the problem has existed in Windows 8 too (and has something to do with welcome screen) but the solution I found does not work on Windows 10 (http://answers.microsoft.com/en-us/windows/forum/windows_8-desktop/custom-hotkeys-to-change-input-language-disappear/66d1d89d-e5dc-41e1-a8b3-48d596ab8e11).
Has anyone been able to solve this problem?