customReducers.js

const languages = (state = [], action) => {
  // reducer logic here
};

const translations = (state = {}, action) => {
  // we now have access to language codes from action
  const codes = action.languageCodes;
  // reducer logic here
}

// custom combine reducers
const rootReducer = (state = {}, action: Action) => {
  const languageCodes = state.languages.map(language => language.code);
  return {
    languages: languages(state.languages, action),
    // merge languageCodes with original action object, now you have access in translations reducer
    translations: translations(state.translations, {...action, languageCodes})
  };
};