Add translation support

https://marmelab.com/react-admin/Translation.html#ra-i18n-i18next

Including language selection by the user (when more than 1 locale available):
https://marmelab.com/react-admin/TranslationSetup.html#supporting-multiple-languages

Default to browser locale:
https://marmelab.com/react-admin/TranslationSetup.html#using-the-browser-locale