Thanks Bogac!

Is this solution working for Windows? It is much more verbose but does not need an external file ans should run on all platform (if && is supported on windows CMD, elsewhere you could simply replace && by ; in the latest line)

"scripts": {
    "build-i18n:fr": "ng build -o dist/fr --aot -prod --bh /fr/ --i18n-file=src/i18n/messages.fr.xlf --i18n-format=xlf --locale=fr",
    "build-i18n:es": "ng build -o dist/es --aot -prod --bh /es/ --i18n-file=src/i18n/messages.es.xlf --i18n-format=xlf --locale=es",
    "build-i18n:en": "ng build -o dist/en --aot -prod --bh /en/ --i18n-file=src/i18n/messages.en.xlf --i18n-format=xlf --locale=en",
    "build-i18n": "npm run build-i18n:en && npm run build-i18n:es && npm run build-i18n:fr"
  }