Cannot translate the value of keypath 'language_option'. Use the value of keypath as default.

I followed the documentation and read every issue related to this error, but still couldn’t fix it. For some reason it can’t translate it, but it doesn’t tell me why. It only outputs this line into the console: Cannot translate the value of keypath 'language_option'. Use the value of keypath as default.

vue & vue-i18n version

2.6.10, 8.11.2

Full code:

https://github.com/MRVDH/mapping-north-korea/tree/2-Add-multi-language-support/client

Reduced code:

main.js:

import Vue from 'vue';
import Vuetify from 'vuetify';
import VueI18n from 'vue-i18n';
import App from './App';
import router from './router';
import 'vuetify/dist/vuetify.min.css';

Vue.config.productionTip = false;

Vue.use(Vuetify);
Vue.use(VueI18n);

/* eslint-disable no-new */
new Vue({
    i18n: {
        fallbackLocale: 'en'
    },
    el: '#app',
    router,
    components: { App },
    template: '<App/>'
});

Component file:

<template>
    <v-toolbar app>
        <v-btn @click.stop="toggleLanguage()">
            {{ $t('language_option') }}
        </v-btn>
    </v-toolbar>
</template>

<i18n>
{
  "en": {
    "language_option": "한국어"
  },
  "ko": {
    "language_option": "English"
  }
}
</i18n>

<script>
export default {
    name: 'Example',
    data () {
        return {
            locale: 'en'
        };
    },
    methods: {
        toggleLanguage: function () {
            console.log('toggle!');
        }
    }
};
</script>

[alexeigs]

I still face the same issue despite the steps described. Warnings are not silenced and I keep getting the following, when locale en is active. No issues when de is active.

[vue-i18n] Value of key 'Hi, nice to meet you' is not a string! 2 vue-i18n.esm.js:32
[vue-i18n] Cannot translate the value of keypath 'Hi, nice to meet you'. Use the value of keypath as default.

Setup:

const i18n = new VueI18n({
  locale: 'en',
  fallbackLocale: 'en',
  formatFallbackMessages: true,
  silentFallbackWarn: true,
  messages
})

en.ts

export default {
  Common: {
    about_us: 'About us',
  }
}

de.ts

export default {
  Common: {
    about_us: 'Über uns',
  },
  'Hi, nice to meet you': 'Hallo, schön dich kennenzulernen'
}

[Silbad]

Oh sorry before return of data :

    data: function() {
        this.$i18n.locale = 'en';
        return {
            locale: 'en'
        };
    },