Add i18n capabilities for custom templates.

For example: The Sphinx reference documentation in doc directory provides
sphinx.pot file from ``doc/_templates/*.html`` by ``make gettext``.

tests/roots/test-intl/_templates/index.html

@@ -0,0 +1,5 @@
+{% extends "layout.html" %}
+{% block body %}
+  <h1>{{ _('Welcome') }}</h1>
+  <p>{%trans%}Sphinx {{ version }}{%endtrans%}</p>
+{% endblock %}

tests/roots/test-intl/conf.py

@@ -5,3 +5,6 @@ import sys, os
 project = 'Sphinx intl <Tests>'
 source_suffix = '.txt'
 keep_warnings = True
+templates_path = ['_templates']
+html_additional_pages = {'index': 'index.html'}
+release = version = '2013.120'

tests/roots/test-intl/sphinx.po

@@ -0,0 +1,23 @@
+# SOME DESCRIPTIVE TITLE.
+# Copyright (C) 2012, foof
+# This file is distributed under the same license as the foo package.
+# FIRST AUTHOR <EMAIL@ADDRESS>, YEAR.
+#
+#, fuzzy
+msgid ""
+msgstr ""
+"Project-Id-Version: sphinx 1.0\n"
+"Report-Msgid-Bugs-To: \n"
+"POT-Creation-Date: 2012-11-22 08:28\n"
+"PO-Revision-Date: YEAR-MO-DA HO:MI+ZONE\n"
+"Last-Translator: FULL NAME <EMAIL@ADDRESS>\n"
+"Language-Team: LANGUAGE <LL@li.org>\n"
+"MIME-Version: 1.0\n"
+"Content-Type: text/plain; charset=UTF-8\n"
+"Content-Transfer-Encoding: 8bit\n"
+
+msgid "Welcome"
+msgstr "WELCOME"
+
+msgid "Sphinx %(version)s"
+msgstr "SPHINX %(version)s"