[Vuejs]-How to intercept i18n prefix change?

You could use the onBeforeLanguageSwitch callback.

From the official documentation

Called before the app’s locale is switched. Can be used to override the new locale by returning a new locale code.

So you could create a plugin that handles the logic for you. For example (as in the official documentation):

/plugins/i18n.js:

export default function ({ app }) {
  // onBeforeLanguageSwitch called right before setting a new locale
  app.i18n.onBeforeLanguageSwitch = (oldLocale, newLocale, isInitialSetup, context) => {
    console.log(oldLocale, newLocale, isInitialSetup)
  }
  // onLanguageSwitched called right after a new locale has been set
  app.i18n.onLanguageSwitched = (oldLocale, newLocale) => {
    console.log(oldLocale, newLocale)
  }
}