feat(app): support locale plural rules (#40370)

Co-authored-by: Luke Parker <10430890+Hona@users.noreply.github.com>

packages/storybook/.storybook/mocks/app/context/language.ts

@@ -123,6 +123,8 @@ const dict: Record<string, string> = {
   "prompt.example.25": "What should we test next?",
 }
 
+const plurals = new Intl.PluralRules("en-US")
+
 function render(template: string, params?: Record<string, unknown>) {
   if (!params) return template
   return template.replace(/\{\{([^}]+)\}\}/g, (_, key: string) => {
@@ -140,5 +142,9 @@ export function useLanguage() {
     t(key: string, params?: Record<string, unknown>) {
       return render(dict[key] ?? key, params)
     },
+    plural(key: string, count: number, params?: Record<string, unknown>) {
+      const value = dict[`${key}.${plurals.select(count)}`] ?? dict[`${key}.other`] ?? key
+      return render(value, { ...params, count })
+    },
   }
 }