Language switcher. Taken from: https://haali.su/winutils/ Eventually will rewrite this to PowerShell script.

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=VK_APPS;

LRESULT CALLBACK KbdHook(int nCode,WPARAM wParam,LPARAM lParam) {
  if (nCode<0)
    return CallNextHookEx(g_khook,nCode,wParam,lParam);
  if (nCode==HC_ACTION) {
    KBDLLHOOKSTRUCT   *ks=(KBDLLHOOKSTRUCT*)lParam;
    if (ks->vkCode==g_key) {
      if (wParam==WM_KEYDOWN) {
	HWND 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(HWND hWnd,UINT uMsg,UINT_PTR idEvent,DWORD dwTime) {
  if (WaitForSingleObject(g_hEvent,0)==WAIT_OBJECT_0)
    PostQuitMessage(0);
}

void xMain() {
  MSG	  msg;
  DWORD	  sz;
  BOOL	  fQuit=FALSE;
  TCHAR	  *cmd;

  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);
}

Mac OS logic uses ~500ms time before firing caps lock event and language switch happens on keyup

Partially implemented this in C# (and Powershell as well), but not published it yet. This is gonna be part of my Universal Utility project.
On request, I can extract this to separate app/script.