SharePoint has a machine translation service that can be used to translate certain documents and phrases. Out of the box, it can be used by Variations to translate pages, and by the Managed Metadata Service to translate terms in the term stores. Beyond that, it is available through the API, and PointFire products make use of it.
For the past several months, Microsoft has been introducing a far superior machine translation engine. So far it is available in English, French, Arabic, Chinese simplified (Mandarin), German, Italian, Japanese, Korean, Portuguese, Russian, and Spanish.
It is not automatically used unless you specifically configure the machine translation service for it. Simply open a session in the SharePoint Management Shell and issue this commandlet:
Set-SPTranslationServiceApplication "Machine Translation Service" -MachineTranslationCategory generalnn
That's it! In this case the service was named "Machine Translation Service". For examples of the old vs the new engine, see http://translate.ai
SharePoint Online uses the old engine, and there is nothing you can do about it. I don't think you can configure the machine translation service for your tenant. Maybe you can request it nicely or vote it up.