Add react-i18next for multi-language support (#917)

This is a rough start on adding react-i18next. I'll be working on adding
more translatable strings and translations in the coming days. I'm going
to need to wrap class components in HOCs, so let me know if there's
something I should be fixing before doing that. I'm thinking now to keep
the exported class names exactly the same, and rename the existing
classes by prefixing an `I` (for internal). For example:

```
export default class AppToolbar ...
```

becomes

```
class IAppToolbar ...
const AppToolbar = withTranslation()(IAppToolbar);
export default AppToolbar;
```

I'll be able to contribute Japanese strings (I've talked to a couple
people on my team and they'll be happy to help as well), so that's the
language I decided to go with in this PR.

Closes #746

---------

Co-authored-by: Ko Nagase <nagase@georepublic.co.jp>
Co-authored-by: Harel M <harel.mazor@gmail.com>

src/components/InputNumber.tsx

@@ -41,7 +41,7 @@ export default class InputNumber extends React.Component<InputNumberProps, Input
     }
   }
 
-  static getDerivedStateFromProps(props: InputNumberProps, state: InputNumberState) {
+  static getDerivedStateFromProps(props: Readonly<InputNumberProps>, state: InputNumberState) {
     if (!state.editing && props.value !== state.value) {
       return {
         value: props.value,