See src/i18n.js and src/tests/vue.js and the unit test BottleCounter.test.js
This allows you to use jest to unit test vue components without having a complicated setup. This way you don't have to mock the $t() functions but actually use vue-i18n itself.
I made this to explain my setup for https://github.com/kazupon/vue-i18n/issues/323
npm install
npm run serve
npm run build
npm run test
This project is maintained by Ilias Ismanalijev.