New Features 
Vue I18n v9 offers not only Vue 3 support, but this version also is included new features.
Message format syntax 
- See the Literal interpolation
Message functions 
- See the Linked messages
- See the Pluralization
Composition API 
See the Composition API advanced section
Translation Component 
Support for plural prop. You can specify the number of messages to be pluralized.
The below example:
vue
<script setup>
import { useI18n } from 'vue-i18n'
const { t } = useI18n({
  locale: 'en',
  messages: {
    en: {
      message: {
        plural: 'no bananas | {n} banana | {n} bananas'
      }
    }
  }
})
const count = ref(0)
</script>
<template>
  <i18n-t keypath="message.plural" :plural="count">
    <template #n>
      <b>{{ count }}</b>
    </template>
  </i18n-t>
</template>DatetimeFormat Component 
For Datetime localization, since Vue I18n v9, we also offer the DatetimeFormat component like the NumberFormat component.
See the Datetime localization custom formatting
i18n Custom Block 
- See the globalattribute