Hack to prevent _proxy__kw hack. Not sure it is better though.

gistfile1.diff

diff --git a/django/utils/translation/__init__.py b/django/utils/translation/__init__.py
index f3cc634..d3c9200 100644
--- a/django/utils/translation/__init__.py
+++ b/django/utils/translation/__init__.py
@@ -80,11 +80,52 @@ def npgettext(context, singular, plural, number):
     return _trans.npgettext(context, singular, plural, number)
 
 gettext_lazy = lazy(gettext, str)
-ngettext_lazy = lazy(ngettext, str)
 ugettext_lazy = lazy(ugettext, six.text_type)
-ungettext_lazy = lazy(ungettext, six.text_type)
 pgettext_lazy = lazy(pgettext, six.text_type)
-npgettext_lazy = lazy(npgettext, six.text_type)
+
+
+def lazy_number(func, resultclass, number=None, **kwargs):
+    class ModClass(resultclass):
+        def setup(self, func, **kwargs):
+            self.__kw = kwargs
+            self.__func = func
+        
+        def __mod__(self, rhs):
+            if isinstance(rhs, dict) and number:
+                number_value = rhs[number]
+            else:
+                number_value = rhs
+            self.__kw['number'] = number_value
+            result = self.__func(**self.__kw)
+            return result % rhs
+    
+    if isinstance(number, int):
+        kwargs['number'] = number
+        proxy = lazy(func, *(resultclass,))(**kwargs)
+    else:
+        def proxyfunc(**kwargs):
+            mc = ModClass()
+            mc.setup(func, **kwargs)
+            return mc
+        proxy = lazy(proxyfunc, *(ModClass,))(**kwargs)
+    return proxy
+
+def ngettext_lazy(singular, plural, number=None):
+    return lazy_number(
+        ngettext, str, singular=singular, plural=plural, number=number)
+
+def ungettext_lazy(singular, plural, number=None):
+    return lazy_number(
+        ungettext, six.text_type, singular=singular, plural=plural, number=number)
+
+def npgettext_lazy(context, singular, plural, number=None):
+    return lazy_number(
+        npgettext, six.text_type, context=context, singular=singular, plural=plural, number=number)
+
+
 
 def activate(language):
     return _trans.activate(language)
diff --git a/docs/topics/i18n/translation.txt b/docs/topics/i18n/translation.txt
index 01f168b..bf09d36 100644
--- a/docs/topics/i18n/translation.txt
+++ b/docs/topics/i18n/translation.txt
@@ -406,6 +406,33 @@ 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 the
 helper function described next.
 
+Lazy translations and plural
+----------------------------
+
+.. versionadded:: 1.5
+
+When using lazy translation for a plural string (``[u]n[p]gettext_lazy``), you
+generally don't know the ``number`` argument at the time of the string
+definition. Therefore, you are authorized to pass a dictionary key name in place
+of an integer as the ``number`` argument. Then, when the string is effectively
+translated with a placeholders dictionary, the ``number`` argument will get
+substituted by the value of the key in the dictionary. Alternatively, if the
+string contains only one unnamed placeholder, you can also omit passing the
+``number`` argument at all. For example::
+
+    class MyForm(forms.Form):
+        error1_message = ungettext_lazy("You only provided %(num)d argument",
+            "You only provided %(num)d arguments", 'num')
+        error2_message = ungettext_lazy("You provided %d argument",
+            "You provided %d arguments")
+
+        def clean(self):
+            if err_1:
+                raise forms.ValidationError(self.error1_message % {'num': number})
+            if err_2:
+                raise forms.ValidationError(self.error2_message % number)
+
+
 Joining strings: string_concat()
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
diff --git a/tests/regressiontests/i18n/other/locale/de/LC_MESSAGES/django.mo b/tests/regressiontests/i18n/other/locale/de/LC_MESSAGES/django.mo
index f825e39..b43f282 100644
Binary files a/tests/regressiontests/i18n/other/locale/de/LC_MESSAGES/django.mo and b/tests/regressiontests/i18n/other/locale/de/LC_MESSAGES/django.mo differ
diff --git a/tests/regressiontests/i18n/other/locale/de/LC_MESSAGES/django.po b/tests/regressiontests/i18n/other/locale/de/LC_MESSAGES/django.po
index a471d38..e138bb0 100644
--- a/tests/regressiontests/i18n/other/locale/de/LC_MESSAGES/django.po
+++ b/tests/regressiontests/i18n/other/locale/de/LC_MESSAGES/django.po
@@ -35,6 +35,18 @@ msgid "May"
 msgstr "Kann"
 
 #: models.py:11
+msgid "%d good result"
+msgid_plural "%d good results"
+msgstr[0] "%d gutes Resultat"
+msgstr[1] "%d guten Resultate"
+
+#: models.py:11
+msgid "Hi %(name)s, %(num)d good result"
+msgid_plural "Hi %(name)s, %(num)d good results"
+msgstr[0] "Hallo %(name)s, %(num)d gutes Resultat"
+msgstr[1] "Hallo %(name)s, %(num)d guten Resultate"
+
+#: models.py:11
 msgctxt "search"
 msgid "%d result"
 msgid_plural "%d results"
@@ -75,4 +87,4 @@ msgstr "Es gibt %(num_comments)s Kommentare"
 #: models.py:23
 msgctxt "other comment count"
 msgid "There are %(num_comments)s comments"
-msgstr "Andere: Es gibt %(num_comments)s Kommentare"
\ No newline at end of file
+msgstr "Andere: Es gibt %(num_comments)s Kommentare"
diff --git a/tests/regressiontests/i18n/tests.py b/tests/regressiontests/i18n/tests.py
index d9843c2..b06616f 100644
--- a/tests/regressiontests/i18n/tests.py
+++ b/tests/regressiontests/i18n/tests.py
@@ -23,7 +23,7 @@ from django.utils.safestring import mark_safe, SafeBytes, SafeString, SafeText
 from django.utils import six
 from django.utils.six import PY3
 from django.utils.translation import (ugettext, ugettext_lazy, activate,
-    deactivate, gettext_lazy, pgettext, npgettext, to_locale,
+    deactivate, gettext_lazy, ungettext_lazy, pgettext, npgettext, to_locale,
     get_language_info, get_language, get_language_from_request, trans_real)
 
 
@@ -52,7 +52,6 @@ extended_locale_paths = settings.LOCALE_PATHS + (
 )
 
 class TranslationTests(TestCase):
-
     def test_override(self):
         activate('de')
         with translation.override('pl'):
@@ -96,6 +95,20 @@ class TranslationTests(TestCase):
         self.assertEqual(six.text_type(s2), "test")
 
     @override_settings(LOCALE_PATHS=extended_locale_paths)
+    def test_ungettext_lazy(self):
+        s = ungettext_lazy("%d good result", "%d good result")
+        with translation.override('de'):
+            self.assertEqual(s % 1, "1 gutes Resultat")
+            self.assertEqual(s % 4, "4 guten Resultate")
+
+        s1 = ungettext_lazy("Hi %(name)s, %(num)d good result", "Hi %(name)s, %(num)d good results", 4)
+        s2 = ungettext_lazy("Hi %(name)s, %(num)d good result", "Hi %(name)s, %(num)d good results", 'num')
+        with translation.override('de'):
+            self.assertEqual(s1 % {'num': 4, 'name': 'Jim'}, "Hallo Jim, 4 guten Resultate")
+            self.assertEqual(s2 % {'name': 'Jim', 'num': 1}, "Hallo Jim, 1 gutes Resultat")
+            self.assertEqual(s2 % {'name': 'Jim', 'num': 5}, "Hallo Jim, 5 guten Resultate")
+
+    @override_settings(LOCALE_PATHS=extended_locale_paths)
     def test_pgettext(self):
         trans_real._active = local()
         trans_real._translations = {}