Elgg supports multiple languages on the same site. By default, users can select their language. There are plugins out there that try to guess the language automatically.

Search the plugin repo for language selector. It should do what you want.

Mike, as Trajan said, there are already plugins that guess the user's language. Elgg's core supports multilingual interface. We also have experience at Elggdev with creating plugins with multilingual content, like vazco_forum or vazco_news.