For a custom Yireo site, we wanted to have multiple languages, but also each language bound to a specific domain-name. Nothing existed yet for this, so we built it ourselves - and now we are proud to offer an extremely useful Joomla! plugin ... entirely for free. Check out it's features.
Joomla! is not really a multisite application - yes, it allows you to access the same Joomla! site through various domains, but it doesn't allow you to configure things specifically for each domain. For such a thing you would perhaps use multisite extensions like JMS. But in the case of language switching the principle was so simple, we wrote our own plugin for it.
Our plugin allows you to configure language-domain-bindings - a rule that says "Use this domain for this language". When you switch to that language, you will be redirect to that domain. When you switch to that domain, that language is loaded. Simple but extremely useful.
Our own use-case for it has yet to go live. But shortly said, it allows for language-specific promotions to be used across multiple domains. Each domain can than be optimized for SEO - running a Google Adwords campaign will be for a specific language and a specific domain for instance, to easily see which language gives which result.
But the plugin is there for you to try out as well: Get it at our site or on GitHub.