angular.json

"build": {
  ...
  "configurations": {
    ...
    "fr": {
      "aot": true,
      "outputPath": "dist/my-project-fr/",
      "i18nFile": "src/locale/messages.fr.xlf",
      "i18nFormat": "xlf",
      "i18nLocale": "fr",
      ...
    }
  }
},
"serve": {
  ...
  "configurations": {
    ...
    "fr": {
      "browserTarget": "*project-name*:build:fr"
    }
  }
}