i18n.txt

====================
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.

If you don't need internationalization in your app
==================================================

Django's internationalization hooks are on by default, and that means there's a
bit of i18n-related overhead in certain places of the framework. If you don't
use internationalization, you should take the two seconds to set
``USE_I18N = False`` in your settings file. If ``USE_I18N`` is set to
``False``, then Django will make some optimizations so as not to load the
internationalization machinery. See the `documentation for USE_I18N`_.

You'll probably also want to remove ``'django.core.context_processors.i18n'``
from your ``TEMPLATE_CONTEXT_PROCESSORS`` setting.

.. _documentation for USE_I18N: ../settings/#use-i18n

If you do need internationalization: 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.

1. 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 ``ugettext()``. It's
convention to import this as a shorter alias, ``_``, to save typing.

.. note::
    Python's standard library ``gettext`` module installs ``_()`` into the
    global namespace, as an alias for ``gettext()``. In Django, we have chosen
    not to follow this practice, for a couple of reasons:

      1. For international character set (Unicode) support, ``ugettext()`` is
         more useful than ``gettext()``. Sometimes, you should be using
         ``ugettext_lazy()`` as the default translation method for a particular
         file. Without ``_()`` in the global namespace, the developer has to
         think about which is the most appropriate translation function.

      2. The underscore character (``_``) is used to represent "the previous
         result" in Python's interactive shell and doctest tests. Installing a
         global ``_()`` function causes interference. Explicitly importing
         ``ugettext()`` as ``_()`` avoids this problem.

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

    from django.utils.translation import ugettext as _

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

Obviously, you could code this without using the alias. This example is
identical to the previous one::

    from django.utils.translation import ugettext

    def my_view(request):
        output = ugettext("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 ``ugettext()`` 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``) whenever you
have more than a single parameter. 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.ugettext_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.ugettext_lazy()`` to translate
strings lazily -- when the value is accessed rather than when the
``ugettext_lazy()`` function is called.

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

    from django.utils.translation import ugettext_lazy

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

In this example, ``ugettext_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 ``ugettext_lazy``, you can just alias it as
``_`` (underscore), like so::

    from django.utils.translation import ugettext_lazy as _

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

Always use lazy translations in `Django models`_. 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 ugettext_lazy as _

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

.. _Django models: ../model-api/

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

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

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

``ungettext`` 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
----------------

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 %}

If you need to bind more than one expression inside a ``blocktrans`` tag,
separate the pieces with ``and``::

    {% blocktrans with book|title as book_t and author|title as author_t %}
    This is {{ book_t }} by {{ author_t }}
    {% endblocktrans %}

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

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

Internally, all block and inline translations use the appropriate
``ugettext`` / ``ungettext`` call.

Each ``RequestContext`` has access to three 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 (translated into the
      currently active locale).
    * ``LANGUAGE_CODE`` is the current user's preferred language, as a string.
      Example: ``en-us``. (See "How language preference is discovered", below.)
    * ``LANGUAGE_BIDI`` is the current locale's direction. If True, it's a
      right-to-left language, e.g: Hebrew, Arabic. If False it's a
      left-to-right language, e.g: English, French, German etc.


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

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

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.

.. note::
    In this example, the translation infrastructure will be passed the string
    ``"yes,no"``, not the individual strings ``"yes"`` and ``"no"``. The
    translated string will need to contain the comma so that the filter
    parsing code knows how to split up the arguments. For example, a German
    translator might translate the string ``"yes,no"`` as ``"ja,nein"``
    (keeping the comma intact).

.. _Django templates: ../templates_python/

Working with lazy translation objects
-------------------------------------

Using ``ugettext_lazy()`` and ``ungettext_lazy()`` to mark strings in models
and utility functions is a common operation. When you're working with these
objects elsewhere in your code, you should ensure that you don't accidentally
convert them to strings, because they should be converted as late as possible
(so that the correct locale is in effect). This necessitates the use of a
couple of helper functions.

Joining strings: string_concat()
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Standard Python string joins (``''.join([...])``) will not work on lists
containing lazy translation objects. Instead, you can use
``django.utils.translation.string_concat()``, which creates a lazy object that
concatenates its contents *and* converts them to strings only when the result
is included in a string. For example::

    from django.utils.translation import string_concat
    ...
    name = ugettext_lazy(u'John Lennon')
    instrument = ugettext_lazy(u'guitar')
    result = string_concat([name, ': ', instrument])

In this case, the lazy translations in ``result`` will only be converted to
strings when ``result`` itself is used in a string (usually at template
rendering time).

The allow_lazy() decorator
~~~~~~~~~~~~~~~~~~~~~~~~~~

Django offers many utility functions (particularly in ``django.utils``) that
take a string as their first argument and do something to that string. These
functions are used by template filters as well as directly in other code.

If you write your own similar functions and deal with translations, you'll
face the problem of what to do when the first argument is a lazy translation
object. You don't want to convert it to a string immediately, because you might
be using this function outside of a view (and hence the current thread's locale
setting will not be correct).

For cases like this, use the  ``django.utils.functional.allow_lazy()``
decorator. It modifies the function so that *if* it's called with a lazy
translation as the first argument, the function evaluation is delayed until it
needs to be converted to a string.

For example::

    from django.utils.functional import allow_lazy

    def fancy_utility_function(s, ...):
        # Do some conversion on string 's'
        ...
    fancy_utility_function = allow_lazy(fancy_utility_function, unicode)

The ``allow_lazy()`` decorator takes, in addition to the function to decorate,
a number of extra arguments (``*args``) specifying the type(s) that the
original function can return. Usually, it's enough to include ``unicode`` here
and ensure that your function returns only Unicode strings.

Using this decorator means you can write your function and assume that the
input is a proper string, then add support for lazy translation objects at the
end.

2. 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.

.. admonition:: Locale restrictions

    Django does not support localizing your application into a locale for
    which Django itself has not been translated. In this case, it will ignore
    your translation files. If you were to try this and Django supported it,
    you would inevitably see a mixture of translated strings (from your
    application) and English strings (from Django itself). If you want to
    support a locale for your application that is not already part of
    Django, you'll need to make at least a minimal translation of the Django
    core.

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 Portuguese 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``.

If run over your project source tree or your application source tree, it will
do the same, but the location of the locale directory is ``locale/LANG/LC_MESSAGES``
(note the missing ``conf`` prefix).

.. 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
directly concatenated. 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. Due to the way the ``gettext`` tools
    work internally and because we want to allow non-ASCII source strings in
    Django's core and your applications, you **must** use UTF-8 as the encoding
    for your PO file. This means that everybody will be using the same
    encoding, which is important when Django processes the PO files.

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! See `Submitting and maintaining translations`_ for
    the steps to take.

    .. _Submitting and maintaining translations: ../contributing/

3. 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.contrib.sessions.middleware.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 that is named according to your ``LANGUAGE_COOKIE_NAME`` setting. (The default name is ``django_language``, and this setting is new in the Django development version. In Django version 0.96 and before, the cookie's name is hard-coded to ``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 Portuguese
      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. If
      you want to restrict the language selection to a subset of provided
      languages (because your application doesn't provide all those languages),
      set ``LANGUAGES`` to a list of languages. For example::

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

      This example restricts languages that are available for automatic
      selection to German and English (and any sublanguage, like de-ch or
      en-us).

      .. _LANGUAGES setting: ../settings/#languages

    * If you define a custom ``LANGUAGES`` setting, as explained in the
      previous bullet, it's OK to mark the languages as translation strings
      -- but use a "dummy" ``ugettext()`` function, not the one in
      ``django.utils.translation``. You should *never* import
      ``django.utils.translation`` from within your settings file, because that
      module in itself depends on the settings, and that would cause a circular
      import.

      The solution is to use a "dummy" ``ugettext()`` function. Here's a sample
      settings file::

          ugettext = lambda s: s

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

      With this arrangement, ``make-messages.py`` will still find and mark
      these strings for translation, but the translation won't happen at
      runtime -- so you'll have to remember to wrap the languages in the *real*
      ``ugettext()`` in any code that uses ``LANGUAGES`` at runtime.

    * The ``LocaleMiddleware`` can only select languages for which there is a
      Django-provided base translation. If you want to provide translations
      for your application that aren't already in the set of translations
      in Django's source tree, you'll want to provide at least basic
      translations for that language. For example, Django uses technical
      message IDs to translate date formats and time formats -- so you will
      need at least those translations for the system to work correctly.

      A good starting point is to copy the English ``.po`` file and to
      translate at least the technical messages -- maybe the validator
      messages, too.

      Technical message IDs are easily recognized; they're all upper case. You
      don't translate the message ID as with other messages, you provide the
      correct local variant on the provided English value. For example, with
      ``DATETIME_FORMAT`` (or ``DATE_FORMAT`` or ``TIME_FORMAT``), this would
      be the format string that you want to use in your language. The format
      is identical to the format strings used by the ``now`` template tag.

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: ../settings/
.. _middleware documentation: ../middleware/
.. _session: ../sessions/
.. _request object: ../request_response/#httprequest-objects

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.

.. note::

    If you're using manually configured settings, as described in the
    `settings documentation`_, the ``locale`` directory in the project
    directory will not be examined, since Django loses the ability to work out
    the location of the project directory. (Django normally uses the location
    of the settings file to determine this, and a settings file doesn't exist
    if you're manually configuring your settings.)

.. _settings documentation: ../settings/#using-settings-without-setting-django-settings-module

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``.

You can also run ``compile-message.py --settings=path.to.settings`` to make
the compiler process all the directories in your ``LOCALE_PATHS`` setting.

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.

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 ``POST`` method, with a ``language``
parameter set in request. If session support is enabled, the view
saves the language choice in the user's session. Otherwise, it saves the
language choice in a cookie that is by default named ``django_language``.
(The name can be changed through the ``LANGUAGE_COOKIE_NAME`` setting if you're
using the Django development version.)

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

    * Django looks for a ``next`` parameter in the ``POST`` data.
    * If that doesn't exist, or is empty, Django tries the URL in the
      ``Referrer`` 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="post">
    <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>

Translations and JavaScript
===========================

Adding translations to JavaScript poses some problems:

    * JavaScript code doesn't have access to a ``gettext`` implementation.

    * JavaScript code doesn't have access to .po or .mo files; they need to be
      delivered by the server.

    * The translation catalogs for JavaScript should be kept as small as
      possible.

Django provides an integrated solution for these problems: It passes the
translations into JavaScript, so you can call ``gettext``, etc., from within
JavaScript.

The ``javascript_catalog`` view
-------------------------------

The main solution to these problems is the ``javascript_catalog`` view, which
sends out a JavaScript code library with functions that mimic the ``gettext``
interface, plus an array of translation strings. Those translation strings are
taken from the application, project or Django core, according to what you
specify in either the info_dict or the URL.

You hook it up like this::

    js_info_dict = {
        'packages': ('your.app.package',),
    }

    urlpatterns = patterns('',
        (r'^jsi18n/$', 'django.views.i18n.javascript_catalog', js_info_dict),
    )

Each string in ``packages`` should be in Python dotted-package syntax (the
same format as the strings in ``INSTALLED_APPS``) and should refer to a package
that contains a ``locale`` directory. If you specify multiple packages, all
those catalogs are merged into one catalog. This is useful if you have
JavaScript that uses strings from different applications.

You can make the view dynamic by putting the packages into the URL pattern::

    urlpatterns = patterns('',
        (r'^jsi18n/(?P<packages>\S+?)/$', 'django.views.i18n.javascript_catalog'),
    )

With this, you specify the packages as a list of package names delimited by '+'
signs in the URL. This is especially useful if your pages use code from
different apps and this changes often and you don't want to pull in one big
catalog file. As a security measure, these values can only be either
``django.conf`` or any package from the ``INSTALLED_APPS`` setting.

Using the JavaScript translation catalog
----------------------------------------

To use the catalog, just pull in the dynamically generated script like this::

    <script type="text/javascript" src="/path/to/jsi18n/"></script>

This is how the admin fetches the translation catalog from the server. When the
catalog is loaded, your JavaScript code can use the standard ``gettext``
interface to access it::

    document.write(gettext('this is to be translated'));

There is also an ``ngettext`` interface::

    var object_cnt = 1 // or 0, or 2, or 3, ...
    s = ngettext('literal for the singular case',
            'literal for the plural case', object_cnt);

and even a string interpolation function::

    function interpolate(fmt, obj, named);

The interpolation syntax is borrowed from Python, so the ``interpolate``
function supports both positional and named interpolation:

    * Positional interpolation: ``obj`` contains a JavaScript Array object
      whose elements values are then sequentially interpolated in their
      corresponding ``fmt`` placeholders in the same order they appear.
      For example::

        fmts = ngettext('There is %s object. Remaining: %s',
                'There are %s objects. Remaining: %s', 11);
        s = interpolate(fmts, [11, 20]);
        // s is 'There are 11 objects. Remaining: 20'

    * Named interpolation: This mode is selected by passing the optional
      boolean ``named`` parameter as true. ``obj`` contains a JavaScript
      object or associative array. For example::

        d = {
            count: 10
            total: 50
        };

        fmts = ngettext('Total: %(total)s, there is %(count)s object',
        'there are %(count)s of a total of %(total)s objects', d.count);
        s = interpolate(fmts, d, true);

You shouldn't go over the top with string interpolation, though: this is still
JavaScript, so the code has to make repeated regular-expression substitutions.
This isn't as fast as string interpolation in Python, so keep it to those
cases where you really need it (for example, in conjunction with ``ngettext``
to produce proper pluralizations).

Creating JavaScript translation catalogs
----------------------------------------

You create and update the translation catalogs the same way as the other
Django translation catalogs -- with the make-messages.py tool. The only
difference is you need to provide a ``-d djangojs`` parameter, like this::

    make-messages.py -d djangojs -l de

This would create or update the translation catalog for JavaScript for German.
After updating translation catalogs, just run ``compile-messages.py`` the same
way as you do with normal Django translation catalogs.

Specialties of Django translation
==================================

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

    * The string domain is ``django`` or ``djangojs``. This string domain is
      used to differentiate between different programs that store their data
      in a common message-file library (usually ``/usr/share/locale/``). The
      ``django`` domain is used for python and template translation strings
      and is loaded into the global translation catalogs. The ``djangojs``
      domain is only used for JavaScript translation catalogs to make sure
      that those are as small as possible.
    * Django doesn't use ``xgettext`` alone. It uses Python wrappers around
      ``xgettext`` and ``msgfmt``. This is mostly for convenience.