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/FieldSourceLayer.tsx

@@ -3,16 +3,17 @@ import React from 'react'
 import {latest} from '@maplibre/maplibre-gl-style-spec'
 import Block from './Block'
 import InputAutocomplete from './InputAutocomplete'
+import { WithTranslation, withTranslation } from 'react-i18next';
 
-type FieldSourceLayerProps = {
+type FieldSourceLayerInternalProps = {
   value?: string
   onChange?(...args: unknown[]): unknown
   sourceLayerIds?: unknown[]
   isFixed?: boolean
   error?: {message: string}
-};
+} & WithTranslation;
 
-export default class FieldSourceLayer extends React.Component<FieldSourceLayerProps> {
+class FieldSourceLayerInternal extends React.Component<FieldSourceLayerInternalProps> {
   static defaultProps = {
     onChange: () => {},
     sourceLayerIds: [],
@@ -20,8 +21,9 @@ export default class FieldSourceLayer extends React.Component<FieldSourceLayerPr
   }
 
   render() {
+    const t = this.props.t;
     return <Block 
-      label={"Source Layer"} 
+      label={t("Source Layer")} 
       fieldSpec={latest.layer['source-layer']}
       data-wd-key="layer-source-layer"
       error={this.props.error}
@@ -35,3 +37,6 @@ export default class FieldSourceLayer extends React.Component<FieldSourceLayerPr
     </Block>
   }
 }
+
+const FieldSourceLayer = withTranslation()(FieldSourceLayerInternal);
+export default FieldSourceLayer;