Build a multilingual site
Run the site locally
npm startVisit / for English, since default-language permalinks are unprefixed, and /nl/ for Dutch. View source on either one and you should find all three alternates, en, nl and x-default, which Baseline emits because both pages share translationKey: 'home'. Then open /about/ and find none, for the reason above.
Check the <html lang> while you are in there. English pages say en, Dutch say nl, and neither of them says so because you typed it on the page.
Dev writes to dist/ as it goes, so the sitemap index at /sitemap.xml and the per-language sitemaps under dist/en/ and dist/nl/ are readable while the server is still up.
Production build and inspect output
Give the build a real origin, the same way run it, then build it did:
BASELINE_URL=https://www.example.com/ npm run builddist/sitemap.xml is no longer a list of pages. It is a sitemap index, naming dist/en/sitemap.xml and dist/nl/sitemap.xml, each of which lists its own language. And every hreflang link in every page now points at the origin you passed rather than localhost.
That is the whole of it. One Eleventy build, two languages, and the only per-language things you wrote by hand were a permalink prefix and two one-line data files.
Two languages was the last thing this tutorial had to show you. Where to go next is the map out of here.