Define exact map using instance as reference.

locale.js

/* @flow */

const messages = {
  "a": "A",
  "b": "B",
}

type Locale_ = typeof messages
type Key = $Keys<Locale_>

type Locale = {
    ...Locale_,
    ...{ [ Key ]: string }
}

const de: Locale = {
  "a": "A",
//  "b": "B", // Error: property `b`. Property not found in Locale object literal
}

const fr: Locale = {
  "a": "A",
  "b": "B",
  "c": "C" // Error: property `c`. Property not found in...{ [ Key ]: string } object literal
}