VoiceOver is supposed to use the app's language. I can change the language with my own settings inside the app. It is done with NSLocalizedString (https://nshipster.com/nslocalizedstring/). My problem is that I don't know what to do with the text automatically pasted by Apple when something is adjustable. I want my sliders to be accessible in appropriate language and behave as adjustable. To the note: You're right that It isn't. I quickly chose an option that had something to do with accessibility. I'm new to Apple and I find it really strange as I can't edit neither my post nor my tags.

Thanks! To be sure we understand one another: In Settings / Accessibility / VoiceOver / Speech / ROTOR LANGUAGES I have English (Samantha), Dutch (Xander) and German (Anna). In ... / VoiceOver / Rotor I have Language checked and disabled from unchecking. The bug is still here! In Settings / "My App" / PREFERRED LANGUAGE I have English and the language of my iPhone is German. The bug is still here! I'm new to iOS development but if a thing has the property accessibilityLanguage set to "en" and the reading voice is an English native speaker one, I consider it a bug that I hear also a phrase in German read immediately afterwards with the same English voice. I'll wait a little bit more for your or any other answer but I think I'll have to use the Feedback Assistant.

You've finally sent the sent with Feedback Assistant. The number is FB9852878. Thank you very much!