Support adding additional cultures to LanguageController.GetAllCultures()

[ThomasSkyldahl]

Currently it's very hard to add extra cultures to Umbraco as it uses CultureInfo.GetCultures(CultureTypes.AllCultures) to get the list of OS supported cultures.
but the new CultureInfo system is smart about custom cultures when they are supplied in the known format:
eg: "en-ES" <LANGUGE_CODE>-<COUNTRY_CODE>

So it would be great if the LanguageController.GetAllCultures() for cultures could be extended to include cultures added during umbraco startup.

One example is that "en-ES" that doesn't currently exist in the call to CultureInfo.GetCultures(CultureTypes.AllCultures).

If we create an instance of var englishSpain = new CultureInfo("en-ES");
Every other API around cultures understands our intention, RegionInfo will understand that its "Spain" we are referring to.
And CultureInfo will know that the language is "English (Spain)" and that its parent is "English".

The current way to add cultures is tough and OS dependent as .NET Core and beyond does not support CultureAndRegionInfoBuilder

[ThomasSkyldahl]

Another way to support custom cultures could be to introduce an ICultureService or ICultureRepository that contains a single method IDictionary<string, string> GetAllCultures(); where the default implementation would contain the current logic from LanguageController.GetAllCultures() but it would be replaceable when needed.