[🔥AUDIT🔥] Fix ngettext not returning plural translations. (#2371)

🖍 _This is an audit!_ 🖍

## Summary:
This bug is in prod right now, unfortunately. Some strings are appearing untranslated.

Issue: FEI-6007

## Test plan:
This was actually wrong in the tests!! So I fixed the test in the process.

Author: jeresig

Auditors: #frontend-infra-web, #wonder-blocks

Required Reviewers:

Approved By:

Checks: ✅ Chromatic - Get results on regular PRs (ubuntu-latest, 20.x), ✅ Test / Test (ubuntu-latest, 20.x, 2/2), ✅ Test / Test (ubuntu-latest, 20.x, 1/2), ✅ Lint / Lint (ubuntu-latest, 20.x), ✅ Check build sizes (ubuntu-latest, 20.x), ✅ Chromatic - Build on regular PRs / chromatic (ubuntu-latest, 20.x), ✅ Publish npm snapshot (ubuntu-latest, 20.x), ⏭️  Chromatic - Skip on Release PR (changesets), ✅ Prime node_modules cache for primary configuration (ubuntu-latest, 20.x), ⏭️  dependabot, ✅ Check for .changeset entries for all changed files (ubuntu-latest, 20.x), ✅ gerald

Pull Request URL: #2371

Author: jeresig

.changeset/fresh-vans-love.md

@@ -0,0 +1,5 @@
+---
+"@khanacademy/wonder-blocks-i18n": patch
+---
+
+Fix ngettext not returning plural translations.

packages/wonder-blocks-i18n/src/functions/__tests__/i18n.test.ts

@@ -139,7 +139,7 @@ describe("i18n", () => {
             const result = ngettext("Singular", "Plural", 0);
 
             // Assert
-            expect(result).toMatchInlineSnapshot(`"Plural"`);
+            expect(result).toMatchInlineSnapshot(`"arrrr mateys"`);
         });
 
         it("doNotTranslate should not translate", () => {

packages/wonder-blocks-i18n/src/functions/i18n.ts

@@ -210,7 +210,7 @@ export const ngettext: ngettextOverloads = (
         typeof singular === "object"
             ? singular
             : {
-                  lang: "en",
+                  lang: getLocale(),
                   // We know plural is a string if singular is not a config object
                   messages: [singular, plural as any],
               };