Select either the Microsoft or Google API menu choice. You decide!
In the unlikely event that you already have a Microsoft Translator API credentials, enter them here and click Save. If not, then click the button "Learn how to get a Microsoft Translator API account". That will redirect you to a Microsoft web page explaining how to get a Bing API key. You will need to enter both the Bing Client Id and Bing Client Secret values. Click the Save button to test your credentials with Bing.
As of December 1st, 2014, the URL to go to is: https://datamarket.azure.com/dataset/bing/microsofttranslator
You may need to create a login to the Microsoft Azure Marketplace. After logging in, look to the menu on the left, and select the Developer entry:
Click the Register Button to register an application.
Now enter the ClientId and Name fields. You can leave the Client Secret value as it, or change it as desired. Enter a meaningful description. You do not need to enter anything into the Redirect URI field. And the Enable subdomain access can be left as is too.
Click the Create Button. After it has been created, you can use the ClientId and ClientSecret values in Milestone Localizer, to register the Microsoft Translator.