fix: Add missing underscore / dash replace on getPluralIndex

Author: xiCO2k

src/utils/get-plural-index.ts

@@ -6,7 +6,7 @@
  * Copyright (c) 2005-2010 - Zend Technologies USA Inc. (http://www.zend.com)
  */
 export function getPluralIndex(lang: string, number: number): number {
-  switch (lang) {
+  switch (lang.replace('_', '-')) {
     case 'az':
     case 'az-AZ':
     case 'bo':

test/fixtures/lang/zh_TW.json

@@ -0,0 +1,3 @@
+{
+    "Welcome!": "歡迎"
+}

test/translate.test.ts

@@ -72,3 +72,10 @@ it('returns the active lang', async () => {
   await loadLanguageAsync('en');
   expect(getActiveLanguage()).toBe('en');
 })
+
+it('translates to a underscore/dash language', async () => {
+  await global.mountPlugin();
+
+  await loadLanguageAsync('zh_TW');
+  expect(getActiveLanguage()).toBe('zh_TW');
+})