Sitemaps with i18n=True load the whole table in memory

[Julien Palard]

Hi!

It's a bit like: https://code.djangoproject.com/ticket/11572

when enabling i18n on a sitemap, _items uses the following code:

`python

# Create (item, lang_code) tuples for all items and languages.
# This is necessary to paginate with all languages already considered.
items = [

(item, lang_code)
for item in self.items()
for lang_code in self.get_languages_for_item(item)

]

`

The list comprehension loads the whole table times the number of languages in memory.

We tried with a table containing two millions elements and two languages: it does not fit in 16GB of memory and the process gets killed.