Add test for `MacOS.languages`.

Library/Homebrew/test/test_os_mac_language.rb

@@ -2,7 +2,15 @@ require "testing_env"
 require "os/mac"
 
 class OSMacLanguageTests < Homebrew::TestCase
+  LANGUAGE_REGEX = /\A[a-z]{2}(-[A-Z]{2})?(-[A-Z][a-z]{3})?\Z/
+
+  def test_languages_format
+    OS::Mac.languages.each do |language|
+      assert_match LANGUAGE_REGEX, language
+    end
+  end
+
   def test_language_format
-    assert_match(/\A[a-z]{2}(-[A-Z]{2})?\Z/, OS::Mac.language)
+    assert_match LANGUAGE_REGEX, OS::Mac.language
   end
 end