CultureInfo .NET 9 maccatalyst/ios country omission regression

[stephenquan]

I am observing a serious regression introduced in .NET 9 impacting mac-catalyst/ios implementations of CultureInfo whereby the country is dropped from CultureInfo.InstalledUICulture, CultureInfo.CurrentUICulture and CultureInfo.CurrentCulture

Language	Windows/Android	mac/ios (.NET 8)	mac/ios (.NET 9)
English (United States)	en-US	en-US	en
English (United Kingdom)	en-GB	en-GB	en
Chinese (Simplified, China)	zh-CN	zh-Hans-CN	zh
Chinese (Traditional, Taiwan)	zh-TW	zh-Hant-TW	zh

The impact is already noticeable in severe localization regression:

• We are no longer able to differentiate between the Chinese variants
• We no longer have access to currency, numeric and date/time formats
• We aren't loading appropriate localization files for different countries (i.e. China and Taiwan need to use different translation resources)

Link to public reproduction project repository
https://github.com/stephenquan/MauiCultureInfoTest

Version with bug
9.0.40

Is this a regression from previous behavior?
Yes, this used to work in .NET MAUI

Last version that worked well
8.0.100 SR10

Affected platforms
iOS, mac-catalyst

Did you find any workaround?
N/A

Related context
This issue may be related to: #27793 and dotnet/runtime#110430

[stephenquan]

After researching further, this issue appears to be a duplicate of dotnet/runtime#110704 - I will move my observations to there.