You are running the esm-bundler build of vue-i18n. It is recommended to configure your bundler to explicitly replace feature flag globals with boolean literals to get proper tree-shaking in the final bundle.

[dybxin]

You are running the esm-bundler build of vue-i18n. It is recommended to configure your bundler to explicitly replace feature flag globals with boolean literals to get proper tree-shaking in the final bundle.

[Liwoj]

Reduce bundle size with feature build flags

[kazupon]

You can do this with the links in the comments above.

[Liwoj]

And here is example how to do it in Vue CLI app (if you are NOT using vue-cli-plugin-i18n)

// vue.config.js
const webpack = require('webpack')

module.exports = {
  chainWebpack: (config) => {
      config.plugin('vue-i18n-feature-flags').use(webpack.DefinePlugin, [{
        __VUE_I18N_LEGACY_API__: 'false',
        __VUE_I18N_FULL_INSTALL__: 'true',
        __VUE_I18N_PROD_DEVTOOLS__: 'false'
      }])
  }
}

Note that in current release branch (9.1.x) the warning will show even if you use config above (because #514 was not merged into 9.1.x branch)