The System.Globalization.CultureInfo class comes with a number of pre-defined cultures but thankfully Microsoft recognises it hasn't supplied all culture/language combinations (real or imagined) and will allow you to build your own. One way you can do this is using the System.Globalization.CultureAndRegionBuilder class.
We ran into trouble while attempting to localise the westernaustralia.com English-language sites targeted at specific regions (we've got a "global" EN site, a "domestic" AU site, and UK, NZ, and SG variants). .NET 2.0 (and Windows XP/Server 2003) define CultureInfos and locales for all of our language/region combinations except for Singapore; the closest in-built option we could find was zh-SG (which is Chinese/Singapore). Although we could have cheated and used zh-SG, we're also running a number of foreign-language variations of the site; to be explicit, avoid confusion, future-proof this aspect of the site, and--most importantly--to make use of .NET's resource fallback mechanism (from en-SG to en), we decided to define a custom CultureInfo.
While creating a new CultureInfo isn't a difficult task, it's not as easy as supplying an "en-SG" string to the CultureInfo constructor or deriving a new class from the CultureInfo class (you can derive a new CultureInfo from an existing CultureInfo, however).
CultureInfo ci = new CultureInfo ("en-SG"); // This will fail at runtime
internal class MyCultureInfo : CultureInfo // There's an easier way...
MSDN provides a succinct article on building a custom CultureInfo class and trust me, it's really quite easy. The article fails to mention that you need to add a reference to the sysglobl assembly to gain access to the CultureAndRegionBuilder class so as long as you remember that step you should be fine. The sample provided also prefixes the new CultureInfo with "x-" and I think this is a great idea: doing so should avoid any conflict when you move to the next version of the .NET Framework or a new platform. Vista and, presumably, Server 2008, include the en-SG locale so naming our new CultureInfo "x-en-SG" means we can anticipate a smooth transition if the existing wa.com code is ever moved to Windows Server 2008.
You don't need to create and register a new CultureInfo every time your application runs (and you probably don't want to since the CultureInfo is written to the filesystem when its registered and invoking Register () again will fail) so we've built the create/register code into our deployment script. We simply try to unregister the existing CultureInfo, create x-en-SG from scratch based on the en-US CultureInfo and the SG RegionInfo, and register the new CultureInfo. Our English-Singapore .resx files reflect the new CultureInfo and are named as though x-en-SG were an in-built CultureInfo: *.x-en-SG.resx.