Как программно переключить раскладку клавиатуры

Используйте функцию ActivateKeyboardLayout (...), которая позволяет установить язык для текущего потока.

А вот пример на Visual C++ который показывает как использовать эту функцию:

**********************************************************************

// NewWordDlg.cpp : implementation file
#include "stdafx.h"
#include "Diction.h"
#include "DictionSet.h"
#include "NewWordDlg.h"

#ifdef _DEBUG
#define new DEBUG_NEW
#undef THIS_FILE
static char THIS_FILE[] = __FILE__;
#endif

/////////////////////////////////////////////////////////////////////////////
// CNewWordDlg dialog

CNewWordDlg::CNewWordDlg(CDictionSet* prs,CWnd* pParent /*=NULL*/)
 : CDialog(CNewWordDlg::IDD, pParent)
{
//{{AFX_DATA_INIT(CNewWordDlg)
m_strRus = _T("");
m_strAngl = _T("");
//}}AFX_DATA_INIT
m_pSet = prs;
}

void CNewWordDlg::DoDataExchange(CDataExchange* pDX)
{
CDialog::DoDataExchange(pDX);
//{{AFX_DATA_MAP(CNewWordDlg)
DDX_Control(pDX, IDCANCEL, m_ctrlCancel);
DDX_Control(pDX, IDOK, m_ctrlOk);
DDX_Text(pDX, IDC_RUS, m_strRus);
DDX_Text(pDX, IDC_ANGL, m_strAngl);
//}}AFX_DATA_MAP
DDX_Text(pDX, IDC_RUS, m_pSet->m_RusWord);
DDX_Text(pDX, IDC_ANGL, m_pSet->m_AnglWord);
}

BEGIN_MESSAGE_MAP(CNewWordDlg, CDialog)
//{{AFX_MSG_MAP(CNewWordDlg)
ON_EN_SETFOCUS(IDC_ANGL, OnSetfocusAngl)
ON_EN_SETFOCUS(IDC_RUS, OnSetfocusRus)
//}}AFX_MSG_MAP
END_MESSAGE_MAP()

//mmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmm
// CNewWordDlg message handlers
BOOL CNewWordDlg::OnInitDialog() 
{
CDialog::OnInitDialog();
lstrcpy(m_szLangOld, _T("\0"));
GetKeyboardLayoutName(m_szLangOld);
return TRUE;
}


void CNewWordDlg::OnSetfocusAngl() 
{
DWORD dwLang = MAKELANGID(LANG_ENGLISH, SUBLANG_ENGLISH_US);
CString strMl; strMl.Format(_T("00000%x"), dwLang);
WORD mlwLang = LOWORD(dwLang);
dwLang = MAKELONG(mlwLang, mlwLang);
CString strLang; strLang.Format(_T("%x"), dwLang);
TCHAR szLang[10];
lstrcpy(szLang, _T("\0"));
GetKeyboardLayoutName(szLang);
if(strMl != szLang) {
lstrcpy(szLang, strLang);
ActivateKeyboardLayout(LoadKeyboardLayout(szLang, KLF_ACTIVATE |
 KLF_REPLACELANG), KLF_REORDER);
}
}

//mmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmm
void CNewWordDlg::OnSetfocusRus() 
{
DWORD dwLang = MAKELANGID(LANG_RUSSIAN, SUBLANG_DEFAULT);
CString strMl; strMl.Format(_T("00000%x"), dwLang);
WORD mlwLang = LOWORD(dwLang);
dwLang = MAKELONG(mlwLang, mlwLang);
CString strLang; strLang.Format(_T("%x"), dwLang);
TCHAR szLang[10]; 
lstrcpy(szLang, _T("\0"));
GetKeyboardLayoutName(szLang);
if(strMl != szLang) {
lstrcpy(szLang, strLang);
ActivateKeyboardLayout(LoadKeyboardLayout(szLang,
 KLF_ACTIVATE | KLF_REPLACELANG), KLF_REORDER);
}
}

void CNewWordDlg::OnOK() 
{
DWORD dwLang = MAKELANGID(LANG_ENGLISH, SUBLANG_ENGLISH_US);
WORD mlwLang = LOWORD(dwLang);
dwLang = MAKELONG(mlwLang, mlwLang);
CString strLang; strLang.Format(_T("%x"), dwLang);
TCHAR szLang[10];
lstrcpy(szLang, _T("\0"));
GetKeyboardLayoutName(szLang);
CString str = m_szLangOld;
if(str != szLang) {
lstrcpy(szLang, strLang);
ActivateKeyboardLayout(LoadKeyboardLayout(szLang,
 KLF_ACTIVATE | KLF_REPLACELANG), KLF_REORDER);
}
CDialog::OnOK();
}

void CNewWordDlg::OnCancel() 
{
DWORD dwLang = MAKELANGID(LANG_ENGLISH, SUBLANG_ENGLISH_US);
WORD mlwLang = LOWORD(dwLang);
dwLang = MAKELONG(mlwLang, mlwLang);
CString strLang; strLang.Format(_T("%x"), dwLang);
TCHAR szLang[10];
lstrcpy(szLang, _T("\0"));
GetKeyboardLayoutName(szLang);
CString str = m_szLangOld;
if(str != szLang) {
lstrcpy(szLang, strLang);
ActivateKeyboardLayout(LoadKeyboardLayout(szLang,
 KLF_ACTIVATE | KLF_REPLACELANG), KLF_REORDER);
}
CDialog::OnCancel();
}