Question: How can I globally overload error messages getting ,for example, from [Required] attribute? Probably, I should change CultureInfo, but I can't find any tutorials. Language still English as default. Thank you!

Answer: 
A first reference on globalization and localization of ASP.NET Core is <https://docs.asp.net/en/latest/fundamentals/localization.html>. 

There you'll find information about how to translate data annotations and how to configure ASP.NET Core to select the appropriate language / `CultureInfo` for each request.

> 
> DataAnnotations error messages are localized with `IStringLocalizer<T>`.
>  Using the option ResourcesPath = "Resources", the error messages in
>  RegisterViewModel can be stored in either of the following paths:
> 
> 
> Resources/ViewModels.Account.RegisterViewModel.fr.resx
>  Resources/ViewModels/Account/RegisterViewModel.fr.resx
> 
> 
> 

For built-in error messages for identity models (like "This email is already taken.) or basic validation texts you have to do some more configuration. I have documented some work in [ASP.Net Core localization](https://stackoverflow.com/questions/39226053/asp-net-core-localization/39227735#39227735).
You can try using `ErrorMessageResourceName` and `ErrorMessageResourceType`, using resource files to store messages.