Multilanguage website, one or two domains?

I want to register a domain and set up my website but I am not sure if I should get one or two domains, as I am planning on making my content available in multiple languages.

When looking at SEO, what is the best choice in this situation? Should I go for subdomains or just register my site with two different TLD's?

Thanks in advance!

-- EDIT --
I am planning on using wordpress so I could easily use a wordpress multilanguage plugin to make the different languages available on the same tld.