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/FieldSource.tsx

@@ -3,24 +3,26 @@ import React from 'react'
 import latest from '@maplibre/maplibre-gl-style-spec/dist/latest.json'
 import Block from './Block'
 import InputAutocomplete from './InputAutocomplete'
+import { WithTranslation, withTranslation } from 'react-i18next';
 
-type FieldSourceProps = {
+type FieldSourceInternalProps = {
   value?: string
   wdKey?: string
   onChange?(value: string| undefined): unknown
   sourceIds?: unknown[]
   error?: {message: string}
-};
+} & WithTranslation;
 
-export default class FieldSource extends React.Component<FieldSourceProps> {
+class FieldSourceInternal extends React.Component<FieldSourceInternalProps> {
   static defaultProps = {
     onChange: () => {},
     sourceIds: [],
   }
 
   render() {
+    const t = this.props.t;
     return <Block
-      label={"Source"}
+      label={t("Source")}
       fieldSpec={latest.layer.source}
       error={this.props.error}
       data-wd-key={this.props.wdKey}
@@ -33,3 +35,6 @@ export default class FieldSource extends React.Component<FieldSourceProps> {
     </Block>
   }
 }
+
+const FieldSource = withTranslation()(FieldSourceInternal);
+export default FieldSource;