Add language method to OS::Mac.

Library/Homebrew/os/mac.rb

@@ -41,6 +41,10 @@ module OS
       version.to_sym
     end
 
+    def language
+      @language ||= Utils.popen_read("defaults", "read", ".GlobalPreferences", "AppleLanguages").delete(" \n\"()").sub(/,.*/, "")
+    end
+
     # Locates a (working) copy of install_name_tool, guaranteed to function
     # whether the user has developer tools installed or not.
     def install_name_tool

Library/Homebrew/test/test_os_mac_language.rb

@@ -0,0 +1,8 @@
+require "testing_env"
+require "os/mac"
+
+class OSMacLanguageTests < Homebrew::TestCase
+  def test_language_format
+    assert_match %r{\A[a-z]{2}(-[A-Z]{2})?\Z}, OS::Mac.language
+  end
+end