D2C checkout falls back to US/USD for regions missing from Unity's culture data

Description
The IAP SDK derives the country it sends on D2C order creation from .NET System.Globalization. Unity's shipped globalization data resolves only a subset of cultures. Where a player's specific culture is absent, RegionInfo.CurrentRegion falls back to US regardless of the OS-level region, and the SDK sends US / USD on to the payment provider. The player is presented with a US checkout they may have no way to pay through.

This is not specific to one market. Any player whose regional variant is missing from the shipped culture data is affected.

Steps to reproduce

1. On a device with OS region set to an affected country (Nigeria used here), run a project on IAP 5.4.1 with a D2C payment provider configured and local-currency prices in the catalog.
2. Log RegionInfo.CurrentRegion.TwoLetterISORegionName, RegionInfo.CurrentRegion.ISOCurrencySymbol, and CultureInfo.CurrentCulture.Name (requires using System.Globalization).
3. Call ShowPurchaseOption(catalogListing) in the same session.

Actual result

The three values return US, USD, en-US. Checkout renders in USD against the provider's US entity, and local payment methods are not offered.

Expected result

Country resolves to the player's actual region, and the checkout session is created against that region's entity and currency.

Culture data detail

CultureInfo.GetCultures(CultureTypes.SpecificCultures) returns three Nigerian specific cultures: ha-Latn-NG, yo-NG, ig-NG. There is no en-NG. Constructing en-NG yields neutral en, after which RegionInfo.CurrentRegion throws NotImplementedException. yo-NG resolves correctly to NG / NGN, and new RegionInfo("NG") works standalone. So the region data exists; the English regional variant an English-language Nigerian build lands on does not.

Ruled out during investigation

• Payment provider portal configuration. The provider's own sandbox test localizes correctly when a country is selected, and edits to US payment methods reflect immediately on the US checkout.
• Routing rules and Targeting, which resolve location by IP and correctly resolved the player's real country, including a VPN test. Routing is correct and checkout is wrong.
• Apple product currency and StoreKit Storefront. Reproduces on Editor, Windows and Android, where there is no StoreKit at all.
• OS region, browser language, physical location, and a brand-new project, all tested independently.
• Native Unity IAP on the same device returned correctly localized prices, since it reads the platform store's data rather than .NET.

Workaround

None for a multi-market title. Pinning CultureInfo.CurrentCulture to a resolvable culture before initialising the StoreController only works if every player is in one country, and the SDK exposes no per-request country override. For Nigeria the pin would have to be yo-NG, ig-NG or ha-Latn-NG, with unknown knock-on effects on number and date formatting and on the Localization package if it reads CultureInfo.