App.vue

<template>
  <ion-app>
    <ion-router-outlet />
  </ion-app>
</template>

<script lang="ts">
import { IonApp, IonRouterOutlet } from '@ionic/vue';
import { defineComponent } from 'vue';
import { Device, DevicePlugin } from '@capacitor/device';

export default defineComponent({

  name: 'App',
  mounted() {
    const device: DevicePlugin = Device;
    device.getLanguageCode().then((res) => {
      console.log('Default lang', res.value);
      if (res.value.includes('-')) {
        const language = res.value.split('-')[0];
        this.$i18n.locale = language;
      } else {
        this.$i18n.locale = res.value;
      }
    })
  },
  components: {
    IonApp,
    IonRouterOutlet
  }
});
</script>