Do I need a domain for every language?

No. A language without a domain keeps its language code and stays on the domain of your default language. You can start with one domain and add more 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 see a country domain as a slightly stronger local signal, but that is an SEO choice, not a technical one.

Do I need a separate Joomla installation for every domain?

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

Is this duplicate content?

No. Every page lives on exactly one address. The language code of a domain's own language redirects away permanently, and the hreflang tags, the x-default and the canonical all point at the right language domain. How it works goes through all of that step by step.

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 a normal multilingual Joomla site with language codes.

Does it work with the page cache?

Yes. The addresses are already right before a page is cached, and Joomla caches per full address, domain included.

Can one language have two domains?

In the plugin, one language has one domain. Do you own an extra domain, an old name for example? Point it at the right domain with a permanent redirect in your web server.

Is the plugin free?

Yes. The plugin is free and every feature is in it. That is everything about routing: the language per domain, the redirects, the links between the domains, the hreflang tags and the canonical correction. You also get 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 version needs a valid certificate. The domain health check warns you two weeks before one runs out.

Does it change my addresses in any other way?

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