Got a reply on our support ticket that the behaviour is based on the AppStore the user is logged into:
"The entry invoked in your plist is based on the device's storefront rather than on the device's language setting. For instance, if you are logged into the App Store with
an Apple ID tied to the UK store, the uk entry in your plist will be invoked. "
Topic:
App & System Services
SubTopic:
StoreKit
Tags: