====================
Internationalization
====================

Django has full support for internationalization of text in code and templates.
Here's how it works.

Overview
========

The goal of internationalization is to allow a single Web application to offer
its content and functionality in multiple languages.

You, the Django developer, can accomplish this goal by adding a minimal amount
of hooks to your Python code and templates. These hooks are called
**translation strings**. They tell Django: "This text should be translated into
the end user's language, if a translation for this text is available in that
language."

Django takes care of using these hooks to translate Web apps, on the fly,
according to users' language preferences.

Essentially, Django does two things:

    * It lets developers and template authors specify which parts of their apps
      should be translatable.
    * It uses these hooks to translate Web apps for particular users according
      to their language preferences.

How to internationalize your app: in three steps
------------------------------------------------

    1. Embed translation strings in your Python code and templates.
    2. Get translations for those strings, in whichever languages you want to
       support.
    3. Activate the locale middleware in your Django settings.


.. admonition:: Behind the scenes

    Django's translation machinery uses the standard ``gettext`` module that
    comes with Python.

How to specify translation strings
==================================

Translation strings specify "This text should be translated." These strings can
appear in your Python code and templates. It's your responsibility to mark
translatable strings; the system can only translate strings it knows about.

In Python code
--------------

Standard translation
~~~~~~~~~~~~~~~~~~~~

Specify a translation string by using the function ``_()``. (Yes, the name of
the function is the "underscore" character.) This function is available
globally in any Python module; you don't have to import it.

In this example, the text ``"Welcome to my site."`` is marked as a translation
string::

    def my_view(request):
        output = _("Welcome to my site.")
        return HttpResponse(output)

The function ``django.utils.translation.gettext()`` is identical to ``_()``.
This example is identical to the previous one::

    from django.utils.translation import gettext
    def my_view(request):
        output = gettext("Welcome to my site.")
        return HttpResponse(output)

Translation works on computed values. This example is identical to the previous
two::

    def my_view(request):
        words = ['Welcome', 'to', 'my', 'site.']
        output = _(' '.join(words))
        return HttpResponse(output)

Translation works on variables. Again, here's an identical example::

    def my_view(request):
        sentence = 'Welcome to my site.'
        output = _(sentence)
        return HttpResponse(output)

(The caveat with using variables or computed values, as in the previous two
examples, is that Django's translation-string-detecting utility,
``make-messages.py``, won't be able to find these strings. More on
``make-messages`` later.)

The strings you pass to ``_()`` or ``gettext()`` can take placeholders,
specified with Python's standard named-string interpolation syntax. Example::

    def my_view(request, n):
        output = _('%(name)s is my name.') % {'name': n}
        return HttpResponse(output)

This technique lets language-specific translations reorder the placeholder
text. For example, an English translation may be ``"Adrian is my name."``,
while a Spanish translation may be ``"Me llamo Adrian."`` -- with the
placeholder (the name) placed after the translated text instead of before it.

For this reason, you should use named-string interpolation (e.g., ``%(name)s``)
instead of positional interpolation (e.g., ``%s`` or ``%d``). If you used
positional interpolation, translations wouldn't be able to reorder placeholder
text.

Marking strings as no-op
~~~~~~~~~~~~~~~~~~~~~~~~

Use the function ``django.utils.translation.gettext_noop()`` to mark a string
as a translation string without translating it. The string is later translated
from a variable.

Use this if you have constant strings that should be stored in the source
language because they are exchanged over systems or users -- such as strings in
a database -- but should be translated at the last possible point in time, such
as when the string is presented to the user.

Lazy translation
~~~~~~~~~~~~~~~~

Use the function ``django.utils.translation.gettext_lazy()`` to translate
strings lazily -- when the value is accessed rather than when the
``gettext_lazy()`` function is called.

For example, to translate a model's ``help_text``, do the following::

    from django.utils.translation import gettext_lazy

    class MyThing(meta.Model):
        name = meta.CharField(help_text=gettext_lazy('This is the help text'))

In this example, ``gettext_lazy()`` stores a lazy reference to the string --
not the actual translation. The translation itself will be done when the string
is used in a string context, such as template rendering on the Django admin site.

If you don't like the verbose name ``gettext_lazy``, you can just alias it as
``_`` (underscore), like so::

    from django.utils.translation import gettext_lazy as _

    class MyThing(meta.Model):
        name = meta.CharField(help_text=_('This is the help text'))

Always use lazy translations in `Django models`_. And it's a good idea to add
translations for the field names and table names, too. This means writing
explicit ``verbose_name`` and ``verbose_name_plural`` options in the ``META``
class, though::

    from django.utils.translation import gettext_lazy as _

    class MyThing(meta.Model):
        name = meta.CharField(_('name'), help_text=_('This is the help text'))
        class META:
            verbose_name = _('my thing')
            verbose_name_plural = _('mythings')

.. _Django models: http://www.djangoproject.com/documentation/model_api/

Pluralization
~~~~~~~~~~~~~

Use the function ``django.utils.translation.ngettext()`` to specify pluralized
messages. Example::

    from django.utils.translation import ngettext
    def hello_world(request, count):
        page = ngettext('there is %(count)d object', 'there are %(count)d objects', count) % {
            'count': count,
        }
        return HttpResponse(page)

``ngettext`` takes three arguments: the singular translation string, the plural
translation string and the number of objects (which is passed to the
translation languages as the ``count`` variable).

In template code
----------------

Using translations in `Django templates`_ uses two template tags and a slightly
different syntax than in Python code. To give your template access to these
tags, put ``{% load i18n %}`` toward the top of your template.

The ``{% trans %}`` template tag translates a constant string or a variable
content::

    <title>{% trans "This is the title." %}</title>

If you only want to mark a value for translation, but translate it later from a
variable, use the ``noop`` option::

    <title>{% trans "value" noop %}</title>

It's not possible to use template variables in ``{% trans %}`` -- only constant
strings, in single or double quotes, are allowed. If your translations require
variables (placeholders), use ``{% blocktrans %}``. Example::

    {% blocktrans %}This will have {{ value }} inside.{% endblocktrans %}

To translate a template expression -- say, using template filters -- you need
to bind the expression to a local variable for use within the translation
block::

    {% blocktrans with value|filter as myvar %}
    This will have {{ myvar }} inside.
    {% endblocktrans %}

To pluralize, specify both the singular and plural forms with the
``{% plural %}`` tag, which appears within ``{% blocktrans %}`` and
``{% endblocktrans %}``. Example::

    {% blocktrans count list|counted as counter %}
    There is only one {{ name }} object.
    {% plural %}
    There are {{ counter }} {{ name }} objects.
    {% endblocktrans %}

Internally, all block and inline translations use the appropriate
``gettext`` / ``ngettext`` call.

Each ``DjangoContext`` has access to two translation-specific variables:

    * ``LANGUAGES`` is a list of tuples in which the first element is the
      language code and the second is the language name (in that language).
    * ``LANGUAGE_CODE`` is the current user's preferred language, as a string.
      Example: ``en-us``. (See "How language preference is discovered", below.)

If you don't use the ``DjangoContext`` extension, you can get those values with
two tags::

    {% get_current_language as LANGUAGE_CODE %}
    {% get_available_languages as LANGUAGES %}

These tags also require a ``{% load i18n %}``.

Translation hooks are also available within any template block tag that accepts
constant strings. In those cases, just use ``_()`` syntax to specify a
translation string. Example::

    {% some_special_tag _("Page not found") value|yesno:_("yes,no") %}

In this case, both the tag and the filter will see the already-translated
string, so they don't need to be aware of translations.

.. _Django templates: http://www.djangoproject.com/documentation/templates_python/

How to create language files
============================

Once you've tagged your strings for later translation, you need to write (or
obtain) the language translations themselves. Here's how that works.

Message files
-------------

The first step is to create a **message file** for a new language. A message
file is a plain-text file, representing a single language, that contains all
available translation strings and how they should be represented in the given
language. Message files have a ``.po`` file extension.

Django comes with a tool, ``bin/make-messages.py``, that automates the creation
and upkeep of these files.

To create or update a message file, run this command::

    bin/make-messages.py -l de

...where ``de`` is the language code for the message file you want to create.
The language code, in this case, is in locale format. For example, it's
``pt_BR`` for Brazilian and ``de_AT`` for Austrian German.

The script should be run from one of three places:

    * The root ``django`` directory (not a Subversion checkout, but the one
      that is linked-to via ``$PYTHONPATH`` or is located somewhere on that
      path).
    * The root directory of your Django project.
    * The root directory of your Django app.

The script runs over the entire Django source tree and pulls out all strings
marked for translation. It creates (or updates) a message file in the directory
``conf/locale``. In the ``de`` example, the file will be
``conf/locale/de/LC_MESSAGES/django.po``.

.. admonition:: No gettext?

    If you don't have the ``gettext`` utilities installed, ``make-messages.py``
    will create empty files. If that's the case, either install the ``gettext``
    utilities or just copy the English message file
    (``conf/locale/en/LC_MESSAGES/django.po``) and use it as a starting point;
    it's just an empty translation file.

The format of ``.po`` files is straightforward. Each ``.po`` file contains a
small bit of metadata, such as the translation maintainer's contact
information, but the bulk of the file is a list of **messages** -- simple
mappings between translation strings and the actual translated text for the
particular language.

For example, if your Django app contained a translation string for the text
``"Welcome to my site."``, like so::

    _("Welcome to my site.")

...then ``make-messages.py`` will have created a ``.po`` file containing the
following snippet -- a message::

    #: path/to/python/module.py:23
    msgid "Welcome to my site."
    msgstr ""

A quick explanation:

    * ``msgid`` is the translation string, which appears in the source. Don't
      change it.
    * ``msgstr`` is where you put the language-specific translation. It starts
      out empty, so it's your responsibility to change it. Make sure you keep
      the quotes around your translation.
    * As a convenience, each message includes the filename and line number
      from which the translation string was gleaned.

Long messages are a special case. There, the first string directly after the
``msgstr`` (or ``msgid``) is an empty string. Then the content itself will be
written over the next few lines as one string per line. Those strings are
directlyconcatenated. Don't forget trailing spaces within the strings;
otherwise, they'll be tacked together without whitespace!

.. admonition:: Mind your charset

    When creating a ``.po`` file with your favorite text editor, first edit
    the charset line (search for ``"CHARSET"``) and set it to the charset
    you'll be using to edit the content. Generally, utf-8 should work for most
    languages, but ``gettext`` should handle any charset you throw at it.

To reexamine all source code and templates for new translation strings and
update all message files for **all** languages, run this::

    make-messages.py -a

Compiling message files
-----------------------

After you create your message file -- and each time you make changes to it --
you'll need to compile it into a more efficient form, for use by ``gettext``.
Do this with the ``bin/compile-messages.py`` utility.

This tool runs over all available ``.po`` files and creates ``.mo`` files,
which are binary files optimized for use by ``gettext``. In the same directory
from which you ran ``make-messages.py``, run ``compile-messages.py`` like
this::

   bin/compile-messages.py

That's it. Your translations are ready for use.

.. admonition:: A note to translators

    If you've created a translation in a language Django doesn't yet support,
    please let us know! We'll add it to the global list of available languages
    in the global Django settings (``settings.LANGUAGES``).

How Django discovers language preference
========================================

Once you've prepared your translations -- or, if you just want to use the
translations that come with Django -- you'll just need to activate translation
for your app.

Behind the scenes, Django has a very flexible model of deciding which language
should be used -- installation-wide, for a particular user, or both.

To set an installation-wide language preference, set ``LANGUAGE_CODE`` in your
`settings file`_. Django uses this language as the default translation -- the
final attempt if no other translator finds a translation.

If all you want to do is run Django with your native language, and a language
file is available for your language, all you need to do is set
``LANGUAGE_CODE``.

If you want to let each individual user specify which language he or she
prefers, use ``LocaleMiddleware``. ``LocaleMiddleware`` enables language
selection based on data from the request. It customizes content for each user.

To use ``LocaleMiddleware``, add ``'django.middleware.locale.LocaleMiddleware'``
to your ``MIDDLEWARE_CLASSES`` setting. Because middleware order matters, you
should follow these guidelines:

    * Make sure it's one of the first middlewares installed.
    * It should come after ``SessionMiddleware``, because ``LocaleMiddleware``
      makes use of session data.
    * If you use ``CacheMiddleware``, put ``LocaleMiddleware`` after it.

For example, your ``MIDDLEWARE_CLASSES`` might look like this::

    MIDDLEWARE_CLASSES = (
       'django.middleware.sessions.SessionMiddleware',
       'django.middleware.locale.LocaleMiddleware',
       'django.middleware.common.CommonMiddleware',
    )

(For more on middleware, see the `middleware documentation`_.)

``LocaleMiddleware`` tries to determine the user's language preference by
following this algorithm:

    * First, it looks for a ``django_language`` key in the the current user's
      `session`_.
    * Failing that, it looks for a cookie called ``django_language``.
    * Failing that, it looks at the ``Accept-Language`` HTTP header. This
      header is sent by your browser and tells the server which language(s) you
      prefer, in order by priority. Django tries each language in the header
      until it finds one with available translations.
    * Failing that, it uses the global ``LANGUAGE_CODE`` setting.

Notes:

    * In each of these places, the language preference is expected to be in the
      standard language format, as a string. For example, Brazilian is
      ``pt-br``.
    * If a base language is available but the sublanguage specified is not,
      Django uses the base language. For example, if a user specifies ``de-at``
      (Austrian German) but Django only has ``de`` available, Django uses
      ``de``.
    * only languages listed in the LANGUAGES setting can be selected. So if you want
      to restrict the language selection to a subset of provided languages (because
      your appliaction doesn't provide all those languages), just set it to a list
      of languages like this::

          LANGUAGES = (
            ('de', _('German')),
            ('en', _('English')),
          )

      This would restrict the available languages for automatic selection to German
      and English (and any sublanguage of those, like de-ch or en-us).

Once ``LocaleMiddleware`` determines the user's preference, it makes this
preference available as ``request.LANGUAGE_CODE`` for each `request object`_.
Feel free to read this value in your view code. Here's a simple example::

    def hello_world(request, count):
        if request.LANGUAGE_CODE == 'de-at':
            return HttpResponse("You prefer to read Austrian German.")
        else:
            return HttpResponse("You prefer to read another language.")

Note that, with static (middleware-less) translation, the language is in
``settings.LANGUAGE_CODE``, while with dynamic (middleware) translation, it's
in ``request.LANGUAGE_CODE``.

.. _settings file: http://www.djangoproject.com/documentation/settings/
.. _middleware documentation: http://www.djangoproject.com/documentation/middleware/
.. _session: http://www.djangoproject.com/documentation/sessions/
.. _request object: http://www.djangoproject.com/documentation/request_response/#httprequest-objects

The ``set_language`` redirect view
==================================

As a convenience, Django comes with a view, ``django.views.i18n.set_language``,
that sets a user's language preference and redirects back to the previous page.

Activate this view by adding the following line to your URLconf::

    (r'^i18n/', include('django.conf.urls.i18n')),

(Note that this example makes the view available at ``/i18n/setlang/``.)

The view expects to be called via the ``GET`` method, with a ``language``
parameter set in the query string. If session support is enabled, the view
saves the language choice in the user's session. Otherwise, it saves the
language choice in a ``django_language`` cookie.

After setting the language choice, Django redirects the user, following this
algorithm:

    * Django looks for a ``next`` parameter in the query string.
    * If that doesn't exist, or is empty, Django tries the URL in the
      ``Referer`` header.
    * If that's empty -- say, if a user's browser suppresses that header --
      then the user will be redirected to ``/`` (the site root) as a fallback.

Here's example HTML template code::

    <form action="/i18n/setlang/" method="get">
    <input name="next" type="hidden" value="/next/page/" />
    <select name="language">
    {% for lang in LANGUAGES %}
    <option value="{{ lang.0 }}">{{ lang.1 }}</option>
    {% endfor %}
    </select>
    <input type="submit" value="Go" />
    </form>

Using translations in your own projects
=======================================

Django looks for translations by following this algorithm:

    * First, it looks for a ``locale`` directory in the application directory
      of the view that's being called. If it finds a translation for the
      selected language, the translation will be installed.
    * Next, it looks for a ``locale`` directory in the project directory. If it
      finds a translation, the translation will be installed.
    * Finally, it checks the base translation in ``django/conf/locale``.

This way, you can write applications that include their own translations, and
you can override base translations in your project path. Or, you can just build
a big project out of several apps and put all translations into one big project
message file. The choice is yours.

All message file repositories are structured the same way. They are:

    * ``$APPPATH/locale/<language>/LC_MESSAGES/django.(po|mo)``
    * ``$PROJECTPATH/locale/<language>/LC_MESSAGES/django.(po|mo)``
    * All paths listed in ``LOCALE_PATHS`` in your settings file are
      searched in that order for ``<language>/LC_MESSAGES/django.(po|mo)``
    * ``$PYTHONPATH/django/conf/locale/<language>/LC_MESSAGES/django.(po|mo)``

To create message files, you use the same ``make-messages.py`` tool as with the
Django message files. You only need to be in the right place -- in the directory
where either the ``conf/locale`` (in case of the source tree) or the ``locale/``
(in case of app messages or project messages) directory are located. And you
use the same ``compile-messages.py`` to produce the binary ``django.mo`` files that
are used by ``gettext``.

Application message files are a bit complicated to discover -- they need the
``LocaleMiddleware``. If you don't use the middleware, only the Django message
files and project message files will be processed.

Finally, you should give some thought to the structure of your translation
files. If your applications need to be delivered to other users and will
be used in other projects, you might want to use app-specific translations.
But using app-specific translations and project translations could produce
weird problems with ``make-messages``: ``make-messages`` will traverse all
directories below the current path and so might put message IDs into the
project message file that are already in application message files.

The easiest way out is to store applications that are not part of the project
(and so carry their own translations) outside the project tree. That way,
``make-messages`` on the project level will only translate strings that are
connected to your explicit project and not strings that are distributed
independently.

Specialities of Django translation
==================================

If you know ``gettext``, you might note these specialities in the way Django
does translation:

    * The string domain is always ``django``. The string domain is used to
      differentiate between different programs that store their data in a
      common message-file library (usually ``/usr/share/locale/``). In Django's
      case, there are Django-specific locale libraries, so the domain itself
      isn't used. We could store app message files with different names and put
      them, say, in the project library, but we decided against this. With
      message files in the application tree, apps can be distributed more
      easily.
    * Django only uses ``gettext`` and ``gettext_noop``. That's because Django
      always uses ``DEFAULT_CHARSET`` strings internally. There isn't much use
      in using ``ugettext``, because you'll always need to produce utf-8
      anyway.
    * Django doesn't use ``xgettext`` alone. It uses Python wrappers around
      ``xgettext`` and ``msgfmt``. That's mostly for convenience.