['en'],
        messages: {
          en: {
            message: {
              hello: 'hello world!'
            }
          },
          // missing key in 'ja' locale messages ...
          ja: {}
        }
      })

      const app = createApp({
        setup() {
          const { t } = useI18n()

          // Something to do ...
          //

          return { t }
        }
      })
      app.use(i18n)
      app.mount('#app')
    </script>
  </body>
</html>
