linux.rb: define language/languages methods

Library/Homebrew/os/linux.rb

@@ -15,6 +15,18 @@ module OS
       Version::NULL
     end
 
+    def languages
+      @languages ||= [
+        *ARGV.value("language")&.split(","),
+        *ENV["HOMEBREW_LANGUAGES"]&.split(","),
+        *ENV["LANG"]&.slice(/[a-z]+/),
+      ].uniq
+    end
+
+    def language
+      languages.first
+    end
+
     module Xcode
       module_function