Vue-i18n - Cannot read property 'config' of undefined

In your main file where you create the app instance add the i18n as option instead of Vue.use(Vuei18n) like this:

new Vue({
  el: '#app',
  i18n,  // < --- HERE
  store,
  router,
  template: '<App/>',
  components: { App }
})

Put it just after el;

And this should be your lang:

import Vue from 'vue'
import VueI18n from 'vue-i18n'
import en from './en'
import fr from './fr'
import ro from './ro'

Vue.use(VueI18n)

export default new VueI18n({
  locale: 'en',
  fallbackLocale: 'en',
  messages: {
    en, fr, ro
  }
})

Just to add a detail, you MUST construct the new VueI18n after using Vue.use(VueI18n)

Vue.use(VueI18n);
// must be called after vue.use
const i18n = new VueI18n({
    locale: "en",
    fallbackLocale: "en",
    messages: {
        en
    }
});
new Vue({
    el: "#app",
    i18n,
    render: (h) => h(App),
});