Add i18n

The app needs to allow changing the language.

We should use something like https://github.com/i18next/react-i18next, or https://github.com/yahoo/react-intl.

The language should be kept in the Redux store. No need for an UI to change the language at this early stage, but we need the multi language support from the start.

No files or tests are named. Start by inspecting the existing React and Redux setup, then compare react-i18next with react-intl against the requirement to keep the language in Redux. Done means the app has an agreed multilingual foundation and language state, without adding a language-switching UI yet.

Written by the indexing model from the issue text.