extensions: Fix ngettext convenience
It is hooked up to the extension's 'gettext' function rather than 'ngettext'.
Fixes: f59d5236 ("extensions: Add static defineTranslationFunctions() method")
It is hooked up to the extension's 'gettext' function rather than 'ngettext'.
Fixes: f59d5236 ("extensions: Add static defineTranslationFunctions() method")