Enable localised jquery-validation error strings

[webknjaz]

demo: https://open-craft-guild.github.io/open-source-guide/#mc_embed_signup
/cc: @bkeepers

[webknjaz]

(tests failed due to some network issues in Travis CI)

[bkeepers]

@webknjaz it looks like there is a relevant failure now:

- _site/starting-a-project/index.html
  *  External link http://ajax.aspnetcdn.com/ajax/jquery.validate/1.15.0/localization/messages_en.js failed: 404 No error
/home/travis/.rvm/gems/ruby-2.3.3/gems/html-proofer-3.5.0/lib/html-proofer/runner.rb:171:in `print_failed_tests': HTML-Proofer found 1 failure! (RuntimeError)
	from /home/travis/.rvm/gems/ruby-2.3.3/gems/html-proofer-3.5.0/lib/html-proofer/runner.rb:52:in `run'
	from script/html-proofer:17:in `<main>'

[bkeepers]

Thanks @webknjaz!