Be aware that Mendix doesn’t translate your app. Adding a language is adding a extra library were you can add the translation for all captions, labels, maessages etc.
So when you develop your app in English, add an other language, that library of the added language is still empty.
When empty, the other language will be shown instead.
Take a look at the documentation regarding translation: https://docs.mendix.com/refguide/batch-translate
Same Problem here.
I have 2 Language Models. English and German.
My Default Language is English, but my app LogIn Page always starts in Germin and i dont find the problem