diff --git a/reactos/dll/win32/kernel32/misc/lang.c b/reactos/dll/win32/kernel32/misc/lang.c
index 862f42409e4..6ebbf9d562a 100644
--- a/reactos/dll/win32/kernel32/misc/lang.c
+++ b/reactos/dll/win32/kernel32/misc/lang.c
@@ -219,8 +219,7 @@ EnumDateFormatsExA(
             break;
 
         default:
-            // FIXME: Unknown date format
-            SetLastError(ERROR_INVALID_PARAMETER);
+            SetLastError(ERROR_INVALID_FLAGS);
             return FALSE;
     }
     return TRUE;
@@ -288,8 +287,7 @@ EnumDateFormatsExW(
             break;
 
         default:
-            // FIXME: Unknown date format
-            SetLastError(ERROR_INVALID_PARAMETER);
+            SetLastError(ERROR_INVALID_FLAGS);
             return FALSE;
     }
     return TRUE;
@@ -334,7 +332,10 @@ static HANDLE NLS_RegOpenKey(HANDLE hRootKey, LPCWSTR szKeyName)
     InitializeObjectAttributes(&attr, &keyName, OBJ_CASE_INSENSITIVE, hRootKey, NULL);
 
     if (NtOpenKey( &hkey, KEY_ALL_ACCESS, &attr ) != STATUS_SUCCESS)
+    {
+        SetLastError( ERROR_BADDB );
         hkey = 0;
+    }
 
     return hkey;
 }
@@ -2397,6 +2398,8 @@ IsValidLanguageGroup(
     case LGRPID_SUPPORTED:
 
         hKey = NLS_RegOpenKey( 0, szLangGroupsKeyName );
+        if (!hKey)
+            break;
 
         swprintf( szValueName, szFormat, LanguageGroup );
 
@@ -2408,10 +2411,13 @@ IsValidLanguageGroup(
                 bInstalled = TRUE;
         }
 
-        if (hKey)
-            NtClose( hKey );
+        NtClose( hKey );
 
         break;
+
+    default:
+        DPRINT("Invalid flags: %lx\n", dwFlags);
+        return FALSE;
     }
 
     if ((dwFlags == LGRPID_SUPPORTED && bSupported) ||
@@ -2440,7 +2446,7 @@ IsValidLanguageGroup(
  */
 BOOL WINAPI
 IsValidLocale(LCID Locale,
-	      DWORD dwFlags)
+              DWORD dwFlags)
 {
     OBJECT_ATTRIBUTES ObjectAttributes;
     PKEY_VALUE_PARTIAL_INFORMATION KeyInfo;
@@ -2452,6 +2458,7 @@ IsValidLocale(LCID Locale,
     HANDLE KeyHandle;
     PWSTR ValueData;
     NTSTATUS Status;
+    BOOL Installed = FALSE;
 
     DPRINT("IsValidLocale() called\n");
 
@@ -2485,6 +2492,7 @@ IsValidLocale(LCID Locale,
     if (!NT_SUCCESS(Status))
     {
         DPRINT("NtOpenKey() failed (Status %lx)\n", Status);
+        SetLastError(ERROR_BADDB);
         return FALSE;
     }
 
@@ -2526,19 +2534,30 @@ IsValidLocale(LCID Locale,
 
     ValueData = (PWSTR)&KeyInfo->Data[0];
     if ((KeyInfo->Type == REG_SZ) &&
-        (KeyInfo->DataLength == 2 * sizeof(WCHAR)) &&
-        (ValueData[0] == L'1'))
+        (KeyInfo->DataLength == 2 * sizeof(WCHAR)))
     {
-        DPRINT("Locale is supported and installed\n");
-        RtlFreeHeap(RtlGetProcessHeap(), 0, KeyInfo);
-        return TRUE;
+        /* Find out if there is support for the language group
+         * installed, to which this language belongs */
+        KeyHandle = NLS_RegOpenKey(0, szLangGroupsKeyName);
+        if (KeyHandle)
+        {
+            WCHAR Value[2];
+            if (NLS_RegGetDword(KeyHandle, ValueData, (LPDWORD) Value) &&
+                Value[0] == L'1')
+            {
+                Installed = TRUE;
+                DPRINT("Locale is supported and installed\n");
+            }
+
+            NtClose(KeyHandle);
+        }
     }
 
     RtlFreeHeap(RtlGetProcessHeap(), 0, KeyInfo);
 
     DPRINT("IsValidLocale() called\n");
 
-    return FALSE;
+    return Installed;
 }
 
 /*
diff --git a/reactos/dll/win32/kernel32/misc/lcformat.c b/reactos/dll/win32/kernel32/misc/lcformat.c
index 2cbabe8fa5e..57cca52b845 100644
--- a/reactos/dll/win32/kernel32/misc/lcformat.c
+++ b/reactos/dll/win32/kernel32/misc/lcformat.c
@@ -109,7 +109,7 @@ BOOL NLS_isSystemLocale(LCID lcid)
 }
 
 /**************************************************************************
- * NLS_isSystemLocale <internal>
+ * NLS_getDefaultLocale <internal>
  *
  * Return default system or user locale
  */
@@ -1023,7 +1023,7 @@ INT WINAPI GetNumberFormatW(LCID lcid, DWORD dwFlags,
   {
       lcid = NLS_getDefaultLocale(lcid);
   }
-  else if(!IsValidLocale(lcid, 0))
+  else if(!IsValidLocale(lcid, LCID_INSTALLED))
   {
       SetLastError(ERROR_INVALID_PARAMETER);
       return 0;
@@ -1403,8 +1403,17 @@ INT WINAPI GetCurrencyFormatW(LCID lcid, DWORD dwFlags,
           lpCurrencyStr,
           cchOut);
 
+  if(NLS_isSystemLocale(lcid))
+  {
+    lcid = NLS_getDefaultLocale(lcid);
+  }
+  else if(!IsValidLocale(lcid, LCID_INSTALLED))
+  {
+    SetLastError(ERROR_INVALID_PARAMETER);
+    return 0;
+  }
+
   if (!lpszValue || cchOut < 0 || (cchOut > 0 && !lpCurrencyStr) ||
-      !IsValidLocale(lcid, 0) ||
       (lpFormat && (dwFlags || !lpFormat->lpDecimalSep || !lpFormat->lpThousandSep ||
       !lpFormat->lpCurrencySymbol || lpFormat->NegativeOrder > 15 ||
       lpFormat->PositiveOrder > 3)))