First of all, you need to add a reference manually to sysglobl.dll.
sysglobl.dll is a very tiny assembly. It contains only one class CultureAndRegionInfoBuilder

Include the namespace System.Globalization.

Add the following code snippet written in C# for achieving it…..

string UserSystemLocale = System.Globalization.CultureInfo.InstalledUICulture.ToString();
string CurrentSiteLocale = web.Locale.ToString();
using (SPSite site = new SPSite("site url"))
{
    try
    {
         using (SPWeb web = site.OpenWeb())
         {
              if (web != null)
              {
                   web.AllowUnsafeUpdates = true;

                   try
                   {
                        CultureAndRegionInfoBuilder.Unregister(CurrentSiteLocale);
                   }
                   catch(Exception ex)
                  {
                     // Log exception
                  }

                   CultureAndRegionInfoBuilder carib = new CultureAndRegionInfoBuilder(CurrentSiteLocale, CultureAndRegionModifiers.Replacement);
                   carib.LoadDataFromCultureInfo(new CultureInfo(UserSystemLocale));
                   carib.LoadDataFromRegionInfo(new RegionInfo(UserSystemLocale));
                   carib.Register();

                   CultureInfo ci = new CultureInfo(UserSystemLocale);
                   web.Locale = ci;
                   web.Update();

                   web.AllowUnsafeUpdates = false;
              }
         }
    }
    catch (Exception ex)
    {
         // Log exception
     }
  }