[[Pywikibot:Cosmetic changes-append/en]]
Fragment of a discussion from Support/LiquidThreads
My previous answer is wrong. The translation of Pywikibot 7+ does not work as expected. The i18n.twtranslate() function raises that exception even a fallback is given; this is not intentional and is a real bug. I'll fix it soon. See phab:T326470.