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>

i18next-parser.config.ts

@@ -0,0 +1,17 @@
+export default {
+  output: 'src/locales/$LOCALE/$NAMESPACE.json',
+  locales: [ 'ja', 'he' ],
+
+  // Because some keys are dynamically generated, i18next-parser can't detect them.
+  // We add these keys manually, so we don't want to remove them.
+  keepRemoved: true,
+
+  // We use plain English keys, so we disable key and namespace separators.
+  keySeparator: false,
+  namespaceSeparator: false,
+
+  defaultValue: (locale, ns, key) => {
+    // The default value is a string that indicates that the string is not translated.
+    return '__STRING_NOT_TRANSLATED__';
+  }
+}