This DEV-only field was added in 836331ba5f69f2cc53b273ec073fae116db14302 by @sebmarkbage but it's not being used anymore to my knowledge.
static childContextTypes = {
        lang: PropTypes.string,
      };
      getChildContext() {
        return {lang: 'en'};
      }
      render() {
        return <Child />;
      }
    }
    function Child(props, context) {