In traditional Sitecore you can always install custom cultures using .NET Framework (unless you're on PaaS ;).
If you're running .NET Core RenderingHost on a alpine linux based container, you're options are a bit more limited. One way would be to build additional cultures into alpine but it's quite a hassle. You need to build the custom .dat file first etc.
However, it turns out that you can actually create new CultureInfo("en-IS")
without them being installed.
There is however one culprit, these cultures are not included in the GetCultures
method. This method is used internally by Sitecore to validate the that the route contains a valid culture for the localization to work.