[1.5.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 8bc350b3 from master.

Conflicts:
	docs/topics/i18n/translation.txt