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The culture list is provided by underlying culture provider. ca-ES-valencia is present in Windows NLS, but not in ICU CLDR which is the default.
You can switch to NLS to get support from Windows.
To request it be included in CLDR, please go to https://cldr.unicode.org/ . New changes need some time to get included by the OS.

Created: https://unicode-org.atlassian.net/jira/software/c/projects/CLDR/issues/CLDR-18583?jql=project%20%3D%20%22CLDR%22%20ORDER%20BY%20created%20DESC

Does this make it appear on all OS or does it only apply to Windows? I have not been able to check if the same thing happens in other OS.

Thanks!

NLS is Windows only. On other OSes currently there is no way to use globalization component other than ICU, and their behavior is expected to the same with Windows with ICU. Note that ICU versions can differ for different OS versions. You have to wait for updated ICU to be included in updated OS.

Ok, thanks!

Note that it's possible to build your own version of ICU, if you really need it.

Potentially, you could also create a custom culture object with Valencian.

Note that it's possible to build your own version of ICU, if you really need it.

And how can it be done? Sounds complicated.

Potentially, you could also create a custom culture object with Valencian.

You mean what I already put new CultureInfo(“ca-ES-valencia”);?

And how can it be done? Sounds complicated.

I'm not completely sure, and it probably depends on distro. But presumably you'd build icu4c.

You mean what I already put new CultureInfo(“ca-ES-valencia”);?

You have a few options:

1. Clone an existing culture and set some properties

   var ci = new CultureInfo("es-ES");
   var ci = (CultureInfo) ci.Clone();
   ci.DateTimeFormat = new DateTimeFormat(...);
   // etc - although not all properties can be changed this way

2. One of the overrides for GetCultureInfo(...) will retrieve a culture even if the data doesn't exist, at which point you should be able to set properties as if cloned.
3. CultureInfo isn't sealed, and most of the properties are virtual. You can subclass it.

Thank you very much for the information @Clockwork-Muse