Union type as key in interface?

You can use an object type instead of an interface, which are mostly interchangeable:

type IMargin = {
    [key in 'foo' | 'bar']: boolean;
}

This is not necessarily an answer, but I think this may interest others.

You can use a Union Type as a key in a sub-property of an Interface.

export type Language = 'EN' | 'DE' | 'IT';

export interface Document {
  generic: string;
  languages: {
    [key in Language]: string[];
  }
}