How to use Vue i18n translation in .js file?

I'm working on VueCLI. I've created .js file that contains rules for inputs:

import Vue from 'vue'

export const $t = (sign) => Vue.prototype.$t(sign)

function checkMail(email) {
  const re = /^(([^<>()\[\]\\.,;:\s@"]+(\.[^<>()\[\]\\.,;:\s@"]+)*)|(".+"))@((\[[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}\])|(([a-zA-Z\-0-9]+\.)+[a-zA-Z]{2,}))$/ // eslint-disable-line no-eval
  return re.test(String(email).toLowerCase())
}

const rules = {
  gdprRequired: (v) => !!v || $t('registerPage.gdprNotChecked'),
  emailRules: [(v) => checkMail(v) || $t('registerPage.wrongMail')],
}

export default rules

I want to declare rules in external js file and then import it in other vue files. When I do this rule check value properly, but It don't show translated error messeage. I tried export const $t = (sign) => Vue.prototype.$t(sign), bcs similar code worked for nuxt, but here, in VueCLI I got error:

vue.runtime.esm.js?2b0e:619 [Vue warn]: Error in beforeMount hook: "ReferenceError: $t is not defined"

found in

---> <VTextField>
       <VForm>
         <Register>
           <VMain>
             <VApp>
               <Session> at src/layouts/Session/Session.vue
                 <App> at src/App.vue
                   <Root>

and this:

vue.runtime.esm.js?2b0e:1888 ReferenceError: $t is not defined
    at rules.emailRules (rules.js?d86d:21)
    at VueComponent.validate (index.ts?91bf:254)
    at VueComponent.beforeMount (index.ts?91bf:217)
    at invokeWithErrorHandling (vue.runtime.esm.js?2b0e:1854)
    at callHook (vue.runtime.esm.js?2b0e:4219)
    at mountComponent (vue.runtime.esm.js?2b0e:4043)
    at VueComponent.Vue.$mount (vue.runtime.esm.js?2b0e:8415)
    at init (vue.runtime.esm.js?2b0e:3118)
    at createComponent (vue.runtime.esm.js?2b0e:5978)
    at createElm (vue.runtime.esm.js?2b0e:5925)

Here importing code in 'register.vue':

import rulesConstant from "@/constants/rules";

Computing exported rules to vue variable:

computed: {
    rules() {
      return rulesConstant;
    },
  },

Example use:

<v-checkbox
          class="custom-checkbox"
          :rules="[rules.gdprRequired]"
          v-model="gdprCheck"
        ></v-checkbox>

Can You help me?

from How to use Vue i18n translation in .js file?