It doesn't do point to point translations from Language A into Language B. It translates from A into English and then English into Language B.
Who knew?
The people at ppcorn.com put it thus:
It Does Not Translate One Language to Another. This might sound totally crazy – not to mention counterintuitive – but Google Translate doesn’t actually translate from one language to another. Instead, it translates the original language to English, and then it translates English to the target language. Though this makes the process easier, it may cause translation errors as English is far from a perfect language.
We'll let pass the theoretically hard-to-swallow notion with which the passage ends: just what might be a "perfect language" anyway?
More fund Google Translate tidbits for your enjoyment here.