angular.json

...
"architect": {
  "build": {
    "builder": "@angular-devkit/build-angular:browser",
    "options": { ... },
    "configurations": {
      "fr": {
        "aot": true,
        "outputPath": "dist/my-project-fr/",
        "i18nFile": "src/locale/messages.fr.xlf",
        "i18nFormat": "xlf",
        "i18nLocale": "fr",
        "i18nMissingTranslation": "error",
      }
    }
  },
  ...
  "serve": {
    "builder": "@angular-devkit/build-angular:dev-server",
    "options": {
      "browserTarget": "my-project:build"
    },
    "configurations": {
      "production": {
        "browserTarget": "my-project:build:production"
      },
      "fr": {
        "browserTarget": "my-project:build:fr"
      }
    }
  }
}