Vue-i18n 'Cannot read property '_t' of undefined at Proxy.Vue.$t'

Ah, embarrassingly simple, I changed

import i18n from './plugins/i18n'

to

import {i18n} from './plugins/i18n'

and all works now.

If anyone came here with the same issue, but is using export default, I had this in my main.js:

import translations from './translations';

new Vue({
    translations,
    ...
}).$mount('#app');

Where translations/index.js loaded the plugin and everything like normal. Apparently you have to name the import 'i18n' for this to work correctly:

import i18n from './translations';

new Vue({
    i18n,
    ...
}).$mount('#app');