Add a language plugin.

This plugin allows to link pages that are translated to another
language. Each resource should have a `language` meta attribute and a
`uuid` meta attribute. Resource that have those two meta attributes
will get a new attribute with the list of other resources in a
different language that have the same `uuid` meta attribute.

It is then possible to use something like this to grab the list of
available languages for a given resource:

{% if resource.translations is defined %}
<p>Other languages</p>
<ul>
  {% for translation in resource.translations %}
    <li><a href="{{ translation.full_url }}">
       {{ languages[translation.meta.language] }}
        </a></li>
  {% endfor %}
</ul>
{% endif %}

(You need a `languages` context in `site.yaml` for this snippet to work:
    context:
      data:
        languages:
          fr: Français
          en: English
)

hyde/ext/plugins/languages.py

@@ -0,0 +1,67 @@
+# -*- coding: utf-8 -*-
+"""
+Contains classes to help manage multi-language pages.
+"""
+
+from hyde.plugin import Plugin
+
+class LanguagePlugin(Plugin):
+    """
+    Each page should be tagged with a language using `language` meta
+    data. Each page should also have an UUID stored in `uuid` meta
+    data. Pages with different languages but the same UUID will be
+    considered a translation of each other.
+
+    For example, consider that you have the following content tree:
+       - `en/`
+       - `fr/`
+
+    In `en/meta.yaml`, you set `language: en` and in `fr/meta.yaml`, you
+    set `language: fr`. In `en/my-first-page.html`, you put something like
+    this::
+         -------
+         uuid: page1
+         -------
+         My first page
+
+     And in `fr/ma-premiere-page.html`, you put something like this::
+         -------
+         uuid: page1
+         -------
+         Ma première page
+
+    You'll get a `translations` attribute attached to the resource
+    with the list of resources that are a translation of this one.
+    """
+
+    def __init__(self, site):
+        super(LanguagePlugin, self).__init__(site)
+        self.languages = {} # Associate a UUID to the list of resources available
+
+    def begin_site(self):
+        """
+        Initialize plugin. Build the language tree for each node
+        """
+        # Build association between UUID and list of resources
+        for node in self.site.content.walk():
+            for resource in node.resources:
+                try:
+                    uuid = resource.meta.uuid
+                    language = resource.meta.language
+                except AttributeError:
+                    continue
+                if uuid not in self.languages:
+                    self.languages[uuid] = []
+                self.languages[uuid].append(resource)
+        # Add back the information about other languages
+        for uuid, resources in self.languages.items():
+            for resource in resources:
+                language = resource.meta.language
+                resource.translations = \
+                    [r for r in resources
+                     if r.meta.language != language]
+                translations = ",".join([t.meta.language for t in resource.translations])
+                self.logger.debug(
+                    "Adding translations for resource [%s] from %s to %s" % (resource,
+                                                                             language,
+                                                                             translations))