What is $i18n property, and why it might be - "is not defined on the instance" ? #340

I never had this problem before, and I can't understand where it comes from, can you help me?
I just took starter https://github.com/davidroyer/nuxt-ssr-firebase and nuxt i18n example, both work fine, but once try to merge it, I get this error. I had the same issue when I try to add vue-i18n in my project (a similar problem with nuxt-i18n also).