Keyboard layout switch on any key, source: http://haali.su/winutils/ + @AHOHNMYC enhancements

lswitch.c

#define _WIN32_WINNT 0x500

#include <windows.h>
#include <tchar.h>

TCHAR	g_prog_dir[MAX_PATH*2];
DWORD	g_prog_dir_len;
HHOOK	g_khook;
HANDLE  g_hEvent;
UINT	g_key;


LRESULT CALLBACK KbdHook(int nCode, WPARAM wParam, LPARAM lParam) {
    HWND hWnd;
    if (nCode == HC_ACTION) {
        KBDLLHOOKSTRUCT* ks = (KBDLLHOOKSTRUCT*)lParam;
        if (ks->vkCode==g_key) {
            /* Negative value => high bit is set => Shift is pressed => collapsing */
            if (GetKeyState(VK_SHIFT)<0)
                goto skip;

            if (wParam==WM_KEYDOWN) {
                hWnd=GetForegroundWindow();
                if (hWnd)
                    PostMessage(hWnd,WM_INPUTLANGCHANGEREQUEST,0,(LPARAM)HKL_NEXT);
            }
            return 1;
        }
    }
skip:
    return CallNextHookEx(g_khook,nCode,wParam,lParam);
}

void  failedx(const TCHAR *msg) {
    MessageBox(NULL,msg,_T("Error"),MB_OK|MB_ICONERROR);
    ExitProcess(1);
}

void  failed(const TCHAR *msg) {
    DWORD		fm;
    TCHAR		*msg1,*msg2;
    const TCHAR	*args[2];

    fm=FormatMessage(FORMAT_MESSAGE_ALLOCATE_BUFFER|FORMAT_MESSAGE_FROM_SYSTEM|
                     FORMAT_MESSAGE_IGNORE_INSERTS,
                     NULL,GetLastError(),0,(LPTSTR)&msg1,0,NULL);
    if (fm==0)
        ExitProcess(1);
    args[0]=msg;
    args[1]=msg1;
    fm=FormatMessage(FORMAT_MESSAGE_ALLOCATE_BUFFER|FORMAT_MESSAGE_FROM_STRING|
                     FORMAT_MESSAGE_ARGUMENT_ARRAY,
                     _T("%1: %2"),0,0,(LPTSTR)&msg2,0,(va_list*)&args[0]);
    if (fm==0)
        ExitProcess(1);
    MessageBox(NULL,msg2,_T("Error"),MB_OK|MB_ICONERROR);
    ExitProcess(1);
}

void CALLBACK TimerCb(__attribute__((unused)) HWND hWnd,
                      __attribute__((unused)) UINT uMsg,
                      __attribute__((unused)) UINT_PTR idEvent,
                      __attribute__((unused)) DWORD dwTime) {
    if (WaitForSingleObject(g_hEvent,0)==WAIT_OBJECT_0)
        PostQuitMessage(0);
}

int __main() {
    MSG	  msg;
    DWORD	  sz;
    BOOL	  fQuit=FALSE;
    TCHAR	  *cmd;

	g_key=VK_APPS;

    cmd=GetCommandLine();
    sz=lstrlen(cmd);
    if (sz>2 && lstrcmp(cmd+sz-2,_T(" q"))==0)
        fQuit=TRUE;
    else {
        UINT    nk=0;
        TCHAR   *qq=cmd+sz;

        while (qq>cmd && qq[-1]>=_T('0') && qq[-1]<=_T('9'))
            --qq;
        while (*qq)
            nk=nk*10+*qq++-_T('0');
        if (nk)
            g_key=nk;
    }

    g_hEvent=CreateEvent(NULL,TRUE,FALSE,_T("HaaliLSwitch"));
    if (g_hEvent==NULL)
        failed(_T("CreateEvent()"));
    if (GetLastError()==ERROR_ALREADY_EXISTS) {
        if (fQuit) {
            SetEvent(g_hEvent);
            goto quit;
        }
        failedx(_T("LSwitch is already running!"));
    }

    if (fQuit)
        failedx(_T("LSwitch is not running!"));

    sz=GetModuleFileName(NULL,g_prog_dir,MAX_PATH);
    if (sz==0)
        failed(_T("GetModuleFileName()"));
    if (sz==MAX_PATH)
        failedx(_T("Module file name is too long."));
    while (sz>0 && g_prog_dir[sz-1]!=_T('\\'))
        --sz;
    g_prog_dir_len=sz;

    if (SetTimer(NULL,0,500,TimerCb)==0)
        failed(_T("SetTimer()"));

    g_khook=SetWindowsHookEx(WH_KEYBOARD_LL,KbdHook,GetModuleHandle(0),0);
    if (g_khook==0)
        failed(_T("SetWindowsHookEx()"));

    while (GetMessage(&msg,0,0,0)) {
        TranslateMessage(&msg);
        DispatchMessage(&msg);
    }

    UnhookWindowsHookEx(g_khook);
quit:
    CloseHandle(g_hEvent);

    ExitProcess(0);
}

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