Provide a 'build' task for all languages (#1678)

Add tasks that build only one of the languages,
similar to the already existing 'build:en' task.

Author: wagurano

Rakefile

@@ -27,18 +27,29 @@ end
 
 namespace :build do
 
-  desc "Build the Jekyll site (`en' language part only)"
-  task :en do
+  def build_subpage(lang)
     require "yaml"
     require "lanyon"
 
     exclude_config = YAML.load_file(CONFIG)["exclude"]
-    exclude_langs  = (LANGUAGES - ["en"]).map {|lang| "#{lang}/" }
+    exclude_langs  = (LANGUAGES - [lang]).map {|x| "#{x}/" }
 
     exclude = exclude_config + exclude_langs
 
     Lanyon.build(exclude: exclude)
   end
+
+  desc "Build the Jekyll site (`lang' language part only)"
+  task :lang do
+    puts 'Please specify one of the valid language codes:'
+    puts LANGUAGES.join(', ') << '.'
+  end
+
+  LANGUAGES.each do |lang|
+    task lang.to_sym do
+      build_subpage(lang)
+    end
+  end
 end
 
 desc "Serve the Jekyll site locally"