Generic functions:

    #region generic regional date formatting functions using .net culture features.
    /// <summary>
    /// 
    /// </summary>
    /// <param name="datestr"></param>
    /// <returns></returns>
    public string UKDateFormat(string datestr)
    {
        return LocalizeDateFormat(datestr, "en-GB");
    }

    /// <summary>
    /// Localize date format using .Net culture features.
    /// </summary>
    /// <param name="datestr">String value containing long date string.</param>
    /// /// <param name="datestr">String value specifying culture, such as en-US or en-GB.</param>
    /// <returns></returns>
    private string LocalizeDateFormat(string datestr, string culture)
    {
        try
        {
            DateTime d = DateTime.Parse(datestr);

            return d.ToString("D", new CultureInfo(culture));
        }
        catch (Exception ex)
        {
            return datestr;
        }
    }
    #endregion

Example Usage:

UKDateFormat(DateTime.Now.ToString());
14 September, 2012

For some commonly used cultures see reference links below containing full list:

References
CSharp-Online (code examples), http://en.csharp-online.net/Localization_Like_the_Pros%E2%80%94Date_formatting
CSharp-Example (list of cultures), http://www.csharp-examples.net/culture-names/