Create custom culture in ASP.NET

You can create a new culture based on an existing culture:

string culture = "en-sg";
string name = "Singaporean English";

CultureInfo cultureInfo = new CultureInfo("en-GB");
RegionInfo regionInfo = new RegionInfo(cultureInfo.Name);

CultureAndRegionInfoBuilder cultureAndRegionInfoBuilder = new CultureAndRegionInfoBuilder(culture, CultureAndRegionModifiers.None);

cultureAndRegionInfoBuilder.LoadDataFromCultureInfo(cultureInfo);
cultureAndRegionInfoBuilder.LoadDataFromRegionInfo(regionInfo);

// Custom Changes
cultureAndRegionInfoBuilder.CultureEnglishName = name;
cultureAndRegionInfoBuilder.CultureNativeName = name;

cultureAndRegionInfoBuilder.Register();

Added: Just checked the references: I have :

using System;
using System.Collections.Generic;
using System.Text;
using System.Globalization;
using System.IO;
using System.Reflection;
using System.Runtime.CompilerServices;

Added (updated, based on comments):

With regards the error message:

The error you're seeing is the result of some resource naming conflict. Check the resource names, these get compiled into dlls to you need to check that the namespace names dont conflict. You can check this using the reflector tool: http://www.red-gate.com/products/reflector/

Here are the steps and code required to create the en-sg culture.

1. Create a console app.
2. Add a reference to sysglobl (C:\Windows\Microsoft.NET\Framework\v2.0.50727\sysglobl.dll)
3. Add the code below to it.
4. Run it on the web server(s) and dev machine(s) as an Administrator.

It will create a culture based on what I found to be its closest match (en-au). I have then overridden names etc to make it unique.

You should only have to run this once. It removes any existing ones before creating this just in case you wish to make any modifications after running it.

public static void Main()
    {
        CultureAndRegionInfoBuilder cib = null;

        try
        {
            Console.Clear();
            Console.WriteLine("Unregister the \"en-SG\" " + "custom culture if it already exists...");
            CultureAndRegionInfoBuilder.Unregister("en-SG");
            Console.WriteLine("The custom culture was unregistered successfully.");
        }
        catch (Exception e)
        {
            Console.WriteLine("Error while unregistering...");
            Console.WriteLine(e);
        }

        try
        {
            cib = new CultureAndRegionInfoBuilder("en-SG", CultureAndRegionModifiers.None);

            // Populate the new CultureAndRegionInfoBuilder object with culture information.
            CultureInfo ci = new CultureInfo("en-AU");
            cib.LoadDataFromCultureInfo(ci);

            // Populate the new CultureAndRegionInfoBuilder object with region information.
            RegionInfo ri = new RegionInfo("SG");
            cib.LoadDataFromRegionInfo(ri);

            cib.CultureEnglishName = "English (Singapore)";
            cib.CultureNativeName = "English (Singapore)";
            cib.IsMetric = true;

            // Display some of the properties of the CultureAndRegionInfoBuilder object.
            Console.WriteLine("CultureName:. . . . . . . . . . {0}", cib.CultureName);
            Console.WriteLine("CultureEnglishName: . . . . . . {0}", cib.CultureEnglishName);
            Console.WriteLine("CultureNativeName:. . . . . . . {0}", cib.CultureNativeName);
            Console.WriteLine("GeoId:. . . . . . . . . . . . . {0}", cib.GeoId);
            Console.WriteLine("IsMetric: . . . . . . . . . . . {0}", cib.IsMetric);
            Console.WriteLine("ISOCurrencySymbol:. . . . . . . {0}", cib.ISOCurrencySymbol);
            Console.WriteLine("RegionEnglishName:. . . . . . . {0}", cib.RegionEnglishName);
            Console.WriteLine("RegionName: . . . . . . . . . . {0}", cib.RegionName);
            Console.WriteLine("RegionNativeName: . . . . . . . {0}", cib.RegionNativeName);
            Console.WriteLine("ThreeLetterISOLanguageName: . . {0}", cib.ThreeLetterISOLanguageName);
            Console.WriteLine("ThreeLetterISORegionName: . . . {0}", cib.ThreeLetterISORegionName);
            Console.WriteLine("ThreeLetterWindowsLanguageName: {0}", cib.ThreeLetterWindowsLanguageName);
            Console.WriteLine("ThreeLetterWindowsRegionName: . {0}", cib.ThreeLetterWindowsRegionName);
            Console.WriteLine("TwoLetterISOLanguageName: . . . {0}", cib.TwoLetterISOLanguageName);
            Console.WriteLine("TwoLetterISORegionName: . . . . {0}", cib.TwoLetterISORegionName);
            Console.WriteLine();

            // Register the custom culture.
            Console.WriteLine("Register the custom culture...");
            cib.Register();

            // Display some of the properties of the custom culture.
            ci = new CultureInfo("en-SG");

            Console.WriteLine("Name: . . . . . . . . . . . . . {0}", ci.Name);
            Console.WriteLine("EnglishName:. . . . . . . . . . {0}", ci.EnglishName);
            Console.WriteLine("NativeName: . . . . . . . . . . {0}", ci.NativeName);
            Console.WriteLine("TwoLetterISOLanguageName: . . . {0}", ci.TwoLetterISOLanguageName);
            Console.WriteLine("ThreeLetterISOLanguageName: . . {0}", ci.ThreeLetterISOLanguageName);
            Console.WriteLine("ThreeLetterWindowsLanguageName: {0}", ci.ThreeLetterWindowsLanguageName);

        }
        catch (Exception e)
        {
            Console.WriteLine(e);
        }
        Console.ReadKey();
    }