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>

cypress/e2e/i18n.cy.ts

@@ -0,0 +1,35 @@
+import { MaputnikDriver } from "./maputnik-driver";
+
+describe("i18n", () => {
+  let { beforeAndAfter, get, when, then } = new MaputnikDriver();
+  beforeAndAfter();
+
+  describe("language detector", () => {
+    it("English", () => {
+      const url = "?lng=en";
+      when.visit(url);
+      then(get.elementByTestId("maputnik-lang-select")).shouldHaveValue("en");
+    });
+
+    it("Japanese", () => {
+      const url = "?lng=ja";
+      when.visit(url);
+      then(get.elementByTestId("maputnik-lang-select")).shouldHaveValue("ja");
+    });
+  });
+
+  describe("language switcher", () => {
+    beforeEach(() => {
+      when.setStyle("layer");
+    });
+
+    it("the language switcher switches to Japanese", () => {
+      const selector = "maputnik-lang-select";
+      then(get.elementByTestId(selector)).shouldExist();
+      when.select(selector, "ja");
+      then(get.elementByTestId(selector)).shouldHaveValue("ja");
+
+      then(get.elementByTestId("nav:settings")).shouldHaveText("スタイル設定");
+    });
+  });
+});

cypress/e2e/maputnik-driver.ts

@@ -131,7 +131,9 @@ export class MaputnikDriver {
         this.helper.when.acceptConfirm();
       }
       // when methods should not include assertions
-      this.helper.get.elementByTestId("toolbar:link").should("be.visible");
+      const toolbarLink = this.helper.get.elementByTestId("toolbar:link")
+      toolbarLink.scrollIntoView();
+      toolbarLink.should("be.visible");
     },
 
     typeKeys: (keys: string) => this.helper.get.element("body").type(keys),