Added rel="alternate" hreflang="XX" to language changer links

AFAICT these attributes are meant to be on <link> tags, but it shouldn't hurt
having them here instead (though if it doesn't help and <link> tags need to be
added, then this commit should probably be reverted).

i2p2www/pages/global/lang.html

 <ul class="languages">
-  <li><a href="{{ change_lang('en') }}"><img src="{{ url_for('static', filename='images/flags/en.png') }}" alt="English" /></a></li>
-  <li><a href="{{ change_lang('es') }}"><img src="{{ url_for('static', filename='images/flags/es.png') }}" alt="Castellano" /></a></li>
-  <li><a href="{{ change_lang('zh') }}"><img src="{{ url_for('static', filename='images/flags/zh.png') }}" alt="Chinese" /></a></li>
-  <li><a href="{{ change_lang('de') }}"><img src="{{ url_for('static', filename='images/flags/de.png') }}" alt="Deutsch" /></a></li>
-  <li><a href="{{ change_lang('fr') }}"><img src="{{ url_for('static', filename='images/flags/fr.png') }}" alt="Français" /></a></li>
-  <li><a href="{{ change_lang('it') }}"><img src="{{ url_for('static', filename='images/flags/it.png') }}" alt="Italiano" /></a></li>
-  <li><a href="{{ change_lang('nl') }}"><img src="{{ url_for('static', filename='images/flags/nl.png') }}" alt="Nederlands" /></a></li>
-  <li><a href="{{ change_lang('ru') }}"><img src="{{ url_for('static', filename='images/flags/ru.png') }}" alt="Russian" /></a></li>
-  <li><a href="{{ change_lang('sv') }}"><img src="{{ url_for('static', filename='images/flags/sv.png') }}" alt="Svenska" /></a></li>
-  <li><a href="{{ change_lang('cs') }}"><img src="{{ url_for('static', filename='images/flags/cs.png') }}" alt="Czech" /></a></li>
-  <li><a href="{{ change_lang('ar') }}"><img src="{{ url_for('static', filename='images/flags/ar.png') }}" alt="Arabic" /></a></li>
-  <li><a href="{{ change_lang('el') }}"><img src="{{ url_for('static', filename='images/flags/el.png') }}" alt="Greek" /></a></li>
+  <li><a href="{{ change_lang('en') }}" rel="alternate" hreflang="en"><img src="{{ url_for('static', filename='images/flags/en.png') }}" alt="English" /></a></li>
+  <li><a href="{{ change_lang('es') }}" rel="alternate" hreflang="es"><img src="{{ url_for('static', filename='images/flags/es.png') }}" alt="Castellano" /></a></li>
+  <li><a href="{{ change_lang('zh') }}" rel="alternate" hreflang="zh"><img src="{{ url_for('static', filename='images/flags/zh.png') }}" alt="Chinese" /></a></li>
+  <li><a href="{{ change_lang('de') }}" rel="alternate" hreflang="de"><img src="{{ url_for('static', filename='images/flags/de.png') }}" alt="Deutsch" /></a></li>
+  <li><a href="{{ change_lang('fr') }}" rel="alternate" hreflang="fr"><img src="{{ url_for('static', filename='images/flags/fr.png') }}" alt="Français" /></a></li>
+  <li><a href="{{ change_lang('it') }}" rel="alternate" hreflang="it"><img src="{{ url_for('static', filename='images/flags/it.png') }}" alt="Italiano" /></a></li>
+  <li><a href="{{ change_lang('nl') }}" rel="alternate" hreflang="nl"><img src="{{ url_for('static', filename='images/flags/nl.png') }}" alt="Nederlands" /></a></li>
+  <li><a href="{{ change_lang('ru') }}" rel="alternate" hreflang="ru"><img src="{{ url_for('static', filename='images/flags/ru.png') }}" alt="Russian" /></a></li>
+  <li><a href="{{ change_lang('sv') }}" rel="alternate" hreflang="sv"><img src="{{ url_for('static', filename='images/flags/sv.png') }}" alt="Svenska" /></a></li>
+  <li><a href="{{ change_lang('cs') }}" rel="alternate" hreflang="cs"><img src="{{ url_for('static', filename='images/flags/cs.png') }}" alt="Czech" /></a></li>
+  <li><a href="{{ change_lang('ar') }}" rel="alternate" hreflang="ar"><img src="{{ url_for('static', filename='images/flags/ar.png') }}" alt="Arabic" /></a></li>
+  <li><a href="{{ change_lang('el') }}" rel="alternate" hreflang="el"><img src="{{ url_for('static', filename='images/flags/el.png') }}" alt="Greek" /></a></li>
 </ul>