Issue 3117 - Restricted "prefixes" attribute in recommendations.xml to language subscriptions

sitescripts/subscriptions/template/recommendations.xml

 <?xml version="1.0"?>
 
 <!--
   - This file is part of the Adblock Plus web scripts,
   - Copyright (C) 2006-2015 Eyeo GmbH
   -
   - Adblock Plus is free software: you can redistribute it and/or modify
   - it under the terms of the GNU General Public License version 3 as
   - published by the Free Software Foundation.
   -
   - Adblock Plus is distributed in the hope that it will be useful,
   - but WITHOUT ANY WARRANTY; without even the implied warranty of
   - MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
   - GNU General Public License for more details.
   -
   - You should have received a copy of the GNU General Public License
   - along with Adblock Plus.  If not, see <http://www.gnu.org/licenses/>.
   -->
 
 <subscriptions>
   {% for subscription in subscriptions|subscriptionSort -%}
   {% if subscription.recommendation -%}
   {% set title, url, complete = subscription.recommendation -%}
   {% set homepage = subscription.homepage or subscription.forum or subscription.blog or subscription.faq or subscription.contact -%}
   <subscription title="{{title}}"
                 specialization="{{subscription.specialization}}"
                 url="{{url}}"
                 homepage="{{homepage}}"
+                {%- if subscription.languages %}
                 prefixes="{{subscription.languages}}"
+                {%- endif %}
                 author="{{subscription.maintainer}}"
                 type="{{subscription.type}}"/>
   {% endif -%}
   {% endfor -%}
 </subscriptions>