The module user by default redirect to "/" allways because is in the Auth controller.
I change redirect()->intended('/') to redirect()->intended('/backend'), and that's work fine for me. The function postLogin not check the config file, that's the problem.
@hgy Thanks for that! We actually ended up needing to go a completely different route for this, abandoned the routes files.. it actually worked out very nicely, your use of settings is one of the pieces of our solution too for the grouping and control of the base path. Good old translatable settings hahaha. We also built a Trait, which we add to any controllers we want to get customized slugs from, it exposes a function we call from the language switcher to pull alternative locales.
I also wanted to share this language switcher file, which works pretty well with custom slugs on a default build using the Blog and Pages modules. This is likely not the best way to do it, but it's a good starting point. Hope this helps someone else!