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/FieldComment.tsx

@@ -2,21 +2,23 @@ import React from 'react'
 
 import Block from './Block'
 import InputString from './InputString'
+import { WithTranslation, withTranslation } from 'react-i18next';
 
-type FieldCommentProps = {
+type FieldCommentInternalProps = {
   value?: string
   onChange(value: string | undefined): unknown
   error: {message: string}
-};
+} & WithTranslation;
 
-export default class FieldComment extends React.Component<FieldCommentProps> {
+class FieldCommentInternal extends React.Component<FieldCommentInternalProps> {
   render() {
+    const t = this.props.t;
     const fieldSpec = {
-      doc: "Comments for the current layer. This is non-standard and not in the spec."
+      doc: t("Comments for the current layer. This is non-standard and not in the spec."),
     };
 
     return <Block
-      label={"Comments"}
+      label={t("Comments")}
       fieldSpec={fieldSpec}
       data-wd-key="layer-comment"
       error={this.props.error}
@@ -25,9 +27,12 @@ export default class FieldComment extends React.Component<FieldCommentProps> {
         multi={true}
         value={this.props.value}
         onChange={this.props.onChange}
-        default="Comment..."
+        default={t("Comment...")}
         data-wd-key="layer-comment.input"
       />
     </Block>
   }
 }
+
+const FieldComment = withTranslation()(FieldCommentInternal);
+export default FieldComment;