Microsoft has changed their authorisation model and there is also a quota now limiting the free translations to 2,000,000 characters per month. The idea is that, first you subscribe to the Microsoft Translator API on Azure Marketplace and then you register your wiki with it. Then you get a Client ID and a Client secret which, according to documentation, are both needed to access the service. So Translate as it is now, cannot use Microsoft Translator service (unless one has an older Bing API Key - like for example in translatewiki.net).
The registration process and how to obtain access token (with sample php code) is described here: http://msdn.microsoft.com/en-us/library/hh454950.aspx
See Also: T43435: Microsoft Translator translation service errors