Here is a good way to configure Vue-i18n so you can use it in your Vuex Store

First, create a file just to configure the internationnalization, we can name it i18n.js, at the root of our project.

import Vue from 'vue';
import VueI18n from 'vue-i18n';
import moment from 'moment';

Vue.use(VueI18n);

// if you are using moment in your project, now is a good time
// to set its locale also
moment.locale(locale);

const i18n = new VueI18n({
  locale, // global variable holding the local value
  messages, // globale variable holding translations
});

export default i18n;

The in your main.js file :

import i18n from './i18n';

// ...

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

And in your Vuex Store (I am using vue-toasted to display messages in a toaster) :

import axios from 'axios';
import Vue from 'vue';
import moment from 'moment';
import i18n from '../../i18n';

export default {
  state : {
    // ...
  },
  mutations : {
    // ...
  },
  actions : {
    doSomething ({ dispatch }, data) {
      return axios.post(`/api/something`, data)
      .then(res => {
        Vue.toasted.success(i18n.tc(`store.success_doing_something`));
      })
      .catch(() => {
        Vue.toasted.error(i18n.tc(`store.error_while_doing_something`));
        return Promise.reject();
      });
    },
  },
};