This fixes a crash with .NET application running with such locales, for example en-DE. The crash happens early in initialization, due to failure to return preferred UI language, which happens because there is no LCID -> locale name conversion for custom locales. It doesn't affect just preferred language, but also system default language, and other derived values.
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v2: ntdll: Do not set system locales that resolve to custom-unspecified.
https://gitlab.winehq.org/wine/wine/-/merge_requests/1417