Ci4 system error localization not working

You need Codeigniter global language translations. It has DE language as well. 
Add this in your composer or download manually in your system

https://github.com/codeigniter4/translations

Already have 'de' translations. and working greate for validation and etc... but not working as expected for error messages for these system error pages. I think this is a bug.

Yes, those error templates are hardcoded in `EN`
Basically i have the same issue rigth now. The cause of the problem is that framework error catching mechanism is throwing back particular (defined) view from config file.
At this point framework should have different mechaninsm - on any defined error code (404, 503 etc) we should have "default" controller or at least methods in BaseController to fully control what data is collected and which view is rendered at the end. This way any developer would have full control of any possible error, load nested or extended view etc.

I have to deal with this, maybe i will manage to get to nice solution or even create some pull request regards this issue