In traditional Sitecore you can always insstall 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)[https://github.com/unicode-org/icu].