metamask-extension/ui/app/ducks/locale/locale.js

import { actionConstants } from '../../store/actions'

export default reduceMetamask

function reduceMetamask (state, action) {
  const localeMessagesState = { ...state.localeMessages }

  switch (action.type) {
    case actionConstants.SET_CURRENT_LOCALE:
      return {
        ...localeMessagesState,
        current: action.value.messages,
      }
    default:
      return localeMessagesState
  }
}