Fix PropTypes check for lang property

Resolves #109.

Author: jakezatecky

src/js/shapes/languageShape.js

@@ -1,7 +1,7 @@
 import PropTypes from 'prop-types';
 
-export default {
+export default PropTypes.shape({
     collapseAll: PropTypes.string.isRequired,
     expandAll: PropTypes.string.isRequired,
     toggle: PropTypes.string.isRequired,
-};
+});