adds translation support via i18n.js

Uses react-i18next library to provide APIs for translational
purposes.
Translation data is loaded from `src/locales/{{lng}}/translation.json`
and is accessed by dot notation.
The data can be translated in two ways:
1. In functional components, a method `t` can be instantiated using the
`useTranslation` method from rect-18next.
2. In class components, a method `t` can be accessed via props while
exporting the component with `withTranslation` method from react-i18next
library.

The syntax will look like `t('data.data')` or `this.props.t('data.data')`

Signed-off-by: Sudhanshu Gautam <me@sudhanshug.com>

src/App.scss

@@ -19,6 +19,10 @@
   color: white;
 }
 
+.language-selector-popper {
+  padding: 15px;
+}
+
 .App-link {
   color: #61dafb;
 }