The content of an app must be understandable by everyone. It is important to set the language to allow assistive technologies to speak the content in the correct language. Also make sure that the behavior of the app is predictable to let users understand what is expected of them. Furthermore, errors must be indicated clearly to allow users to fix them.
It's challenging to learn an app when the navigation on each screen is different.
The language of the app must be set so that the screen reader to read the content in the correct language.
When mistakes are made, it is important to clearly indicate them and to offer assistance.