* In LanguageSelector, use React.forwardRef() to enable users of
  LanguageSelector to obtain a ref to the LanguageSelector button

* Add 2 new instance properties to App: 'languageSelectorRef', a ref
  to the language selector (forwarded to the button, as above), and
  'focusLanguageSelector', a boolean to indicate that focus should be
  set on the language selector

* Add a new language change handler to App that sets
  'focusLanguageSelector' when the language is changed and new code to
  'componentDidUpdate()' to check 'focusLanguageSelector' and set
  focus if requested