Setting up iOS device / simulator to retrieve en_150 as locale

Question

How do iOS apps evaluate to a specific Locale?

Regarding Apple Docs, it follows standardized way (CLDR) with apple customized fallback strategies.

In Swift, I'm trying to figure out how Locale.current.identifier or Objective-C equivalent specifically output:

• en
• en_150
• es
• ...

(Without having a region designator)

How do I configure the simulator or device to achieve this?

What else influences Locale.current?

• preferred language(s) (also the order)
• selected region
• supported languages from your app
• (some other settings like calendar, phonebook collation) these are suffixed with an @

Interesting sources:

https://developer.apple.com/library/archive/documentation/MacOSX/Conceptual/BPInternational/LanguageandLocaleIDs/LanguageandLocaleIDs.html

https://opensource.apple.com/source/ICU/ICU-57131.0.1/icuSources/common/unicode/ualoc.h.auto.html

https://forums.developer.apple.com/thread/71383