working on issue2551184 i found the following:

translationService is only used in cgi.form_parser.py as fallback (which i dont know when this can happen, because client always has self.gettext and self.ngettext - but maybe i am wrong - maybe during tests ?)

could be better to remove it from templating.py and replace it by TranslationService.get_translation()

see attached patch

regards,
marcus