I don’t have any technical input on this issue (it’s way outside of my area of expertise) but I don’t have two general suggestions:
If you don’t get a response here, you might try reposting over in App Frameworks > Cocoa Touch to see if folks over there have any input.
And if that doesn’t work out my recommendation is that you open a DTS tech support incident and discuss that with out localisation expert.
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Quinn “The Eskimo!”
Apple Developer Relations, Developer Technical Support, Core OS/Hardware
let myEmail = "eskimo" + "1" + "@apple.com"
What is the primaryLanguage of your input view controller? (https://developer.apple.com/documentation/uikit/uiinputviewcontroller/1618200-primarylanguage?language=objc)
The value of this property helps the OS decide which preferred languages should be added after enabling your keyboard.
If your keyboard only supports one language, then the primaryLanguage should be changed to accurately reflect the languages it supports. If you're planning on supporting multiple languages, there are a couple ways to do this that are outlined here: https://developer.apple.com/library/archive/documentation/General/Conceptual/ExtensibilityPG/CustomKeyboard.html
Let me know if you have any more questions.