Update the new sandbox page with lots of information on how twig templates and translations work.

Add also proofs of concept for every example given.

templates/sandbox.twig

 {% extends "base.twig" %}
 {% block content %}
-    <p>This page exists as a sandbox to play with twig without affecting anything else. The template is in ./templates.</p>
-    <p>{{ sometext }}</p>
-    <h2>And now for some localization</h2>
-    <p>Locale backend in use: {{ localeBackend }}</p>
-    <p>Original: Hello, Untranslated World!</p>
-    <p>Translated: {% trans 'Hello, Untranslated World!' %}</p>
-    <p>Filtertrans-test: {{ 'Hello, Untranslated World!'|trans }}</p>
-    <p>Current locale set: {{ currentLanguage }}</p>
+    <h1>Sandbox</h1>
+    <p>This page serves as a demonstration of the <strong>new template and translation sub-systems</strong> in
+        SimpleSAMLphp. The page itself is written as a <em>Twig</em> template, which is very similar to other
+        templating languages, stored in the <code>templates</code> directory.</p>
+    <p>Twig templates allow you to print values of variables very easily. For example, the code <code>{{ '{{ ' }}
+            sometext }}</code>
+        will print the following text, contained in the variable <code>sometext</code>:
+    </p>
+    <p><em>{{ sometext }}</em></p>
+    <p>Twig supports setting your own variables, control structures like <code>if</code> clauses and loops.
+        Take a look at the <a href="http://twig.sensiolabs.org/doc/templates.html">Twig documentation for
+        template designers</a> if you want to know more.
+    </p>
+    <h2>Localization</h2>
+    {% set variable = 'Hello, Untranslated World!' %}
+    <p>SimpleSAMLphp lets you choose which translation backend to choose, thanks to the
+        <code>language.i18n.backend</code> configuration option. Three possible values are supported there:
+    </p>
+    <ul>
+        <li><code>SimpleSAMLphp</code>: to keep using the old SimpleSAMLphp translation system. This is the
+            default, and will disappear as an option in SimpleSAMLphp 2.0.</li>
+        <li><code>ext-intl</code>: to use PHP's native <em>gettext</em> implementation. Bear in mind that using this
+            will require you to install the locales of the languages you are planning to use, system-wide.</li>
+        <li><code>gettext/gettext</code>: to use a <em>gettext</em> implementation written entirely in PHP, allowing
+            you to use any locale, no matter if they are installed on the system or not.</li>
+    </ul>
+    <p>Note that <code>gettext/gettext</code> <strong>will become the default</strong> in SimpleSAMLphp 2.0.
+        Currently, you are using the following backend: <code>{{ localeBackend }}</code>.</p>
+    <p>Now, Twig allows you to translate strings in your templates. There are several ways to do that. If you want
+        to translate the following text: <em>Hello, Untranslated World!</em>, you can do it with:</p>
+    <ul>
+        <li><em>Inline trans tags</em>: using <code>{{ '{%' }} trans 'Hello, Untranslated World! %}</code> you would get
+            "{% trans 'Hello, Untranslated World!' %}".</li>
+        <li><em>Expanded trans tags</em>: using <code>{{ '{%' }} trans %}Hello, Untranslated World!{{ '{%' }} endtrans
+                %}</code> you would get "{% trans %}Hello, Untranslated World!{% endtrans %}".</li>
+        <li><em>Filters</em>: using <code>{{ '{{' }} variable|trans }}</code> you would get "{{ variable|trans }}".</li>
+    </ul>
+    <p>Translations support arguments too, so that you can replace parts of the translated string with the contents of
+        variables. Just use placeholders of the form <code>%variable%</code> in the place where the contents of the
+        variables should be placed, and pass an associative array to the <code>trans </code> filter.</p>
+    {% set variable = 'Hello, %who%!' %}
+    {% set who = {'%who%': 'World'} %}
+    <p>If you have a variable with the text "<code>Hello, %who%!</code>" The code <code>{{ '{{' }}
+        variable|trans({'%who%': 'World' }) }}</code> will print "{{ variable|trans({'%who%': 'World'}) }}". The array
+        can also be passed in a variable, so that  <code>{{ '{{' }} variable|trans(who) }}</code> will also output
+        "{{ variable|trans(who) }}". Note that placeholders have names, so order is irrelevant, and can be changed
+        between translations.</p>
+
 {% endblock content %}