This is based on the
CFBundleDevelopmentRegionvalue in your
Info.plist. QA1828 How iOS Determines the Language For Your App discusses this stuff in detail.
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Quinn “The Eskimo!”
Apple Developer Relations, Developer Technical Support, Core OS/Hardware
let myEmail = "eskimo" + "1" + "@apple.com"
This happens because iOS doesn't only looks for the current system language.
It looks for the first supported language in the list of preferred languages.
But when you choose Dutch in system preferences, iOS automatically adds this language to the list of preferred languages.
Thus, when you change language to Polish, Dutch appears the second in the list (after Polish).
And because your app supports Dutch, but not Polish, the app displays in Dutch.