The featurel is exceedingly simple to use. With a document open, simply select the "Tools" option. "Translate Document" appears in the Tools listing and lets you pick which language you'd like to translate the document into.
Supported languages include; Afrikaans, Albanian, Arabic, Belarusian, Bulgarian, Catalan, Chinese, Croatian, Czech, Danish, Dutch, English, Estonian, Filipino, Finnish, French, Galician, German, Greek, Hebrew, Hindi, Hungarian, Icelandic, Indonesian, Irish, Italian, Japanese, Korean, Latvian, Lithuanian, Macedonian, Malay, Maltese, Norwegian, Persian, Polish, Portuguese, Romanian, Russian, Serbian, Slovak, Slovenian, Spanish, Swahili, Swedish, Thai, Turkish, Ukrainian, Vietnamese, Welsh, and Yiddish.
That's not a bad list, and gives most users a head start at getting documents translated into a language that someone, somewhere can understand. The feature is already available in Google Docs.