Add `vi` to available locales (#13542)

* Enable selecting Vietnamese language in Admin Dashboard

* Update settings_helper.rb

Co-authored-by: Eugen Rochko <eugen@zeonfederated.com>

app/helpers/settings_helper.rb

@@ -68,6 +68,7 @@ module SettingsHelper
     tr: 'Türkçe',
     uk: 'Українська',
     ur: 'اُردُو',
+    vi: 'Tiếng Việt',
     'zh-CN': '简体中文',
     'zh-HK': '繁體中文（香港）',
     'zh-TW': '繁體中文（臺灣）',

config/application.rb

@@ -106,6 +106,7 @@ module Mastodon
       :tr,
       :uk,
       :ur,
+      :vi,
       :'zh-CN',
       :'zh-HK',
       :'zh-TW',