Do I need a domain for every language?

No. A language without a domain keeps its language code and is served on the domain of the default site language. You can start with one domain and add others later.

Can two languages share one domain?

Not as their own domain: one domain belongs to one language. Languages without a domain do live together on the default domain, each with its own language code.

Can I use subdomains?

Yes. A domain here is simply a host name, so nl.example.com works exactly like www.example.nl. Search engines treat a country domain as a stronger local signal, but that is an SEO choice, not a technical one.

Do I need a separate Joomla installation per domain?

No, and that is the point. One installation, one administrator, one set of extensions, several domains.

Is this duplicate content?

No. Every page exists on exactly one URL, the language code of a domain's own language redirects away permanently, and the hreflang links, the x-default and the canonical all point at the domain of each language.

What happens on my staging domain?

Nothing at all. A host that is not in your list is left untouched, so staging, localhost and an IP address keep working like an ordinary multilingual Joomla site with language codes.

Does it work with the page cache?

Yes. The URLs are already correct before a page is cached, and Joomla caches per full URL including the host.

Can one language have two domains?

One language has one domain in the plugin. An extra domain you own - an old name, another extension - is best pointed at the right domain with a permanent redirect in your web server.

What does the free edition include?

Everything about routing: the language per domain, the redirects, the links between domains, hreflang and the canonical correction. PRO adds a robots.txt per domain, the language suggestion, the domain health check and staying logged in across domains.

Do I need a certificate for every domain?

Yes, every domain and every www variant needs a valid certificate. The PRO health check warns you two weeks before one expires.

Does it change my URLs in any other way?

No. Only the language code of the domain's own language disappears; the rest of the URL is built by Joomla and your menu structure exactly as before.