[1.6.x] Fixed #21372 -- Corrected docs regarding translating LANGUAGES.

Corrected LANGUAGES documentation on how to translate language
names. Now using django.utils.translation.ugettext_lazy instead
of a dummy gettext() function.

Thanks to Salvatore for the report.

Backport of 8bc350b38516d8c3a14aed113dd3402b9375b75c from master.