Bing Translator is the best freely available machine translation API that I know of. If you're interested in getting set up to use the API to make your own translation enabled apps, follow this guide.
Create an Azure marketplace account1- Go to http://datamarket.azure.com/ and login using a Microsoft live account. If you don't have one, you'll need to create one beforehand. You can do that here.
2- Complete the registration:
3- Once logged in, head on over to the bing translator subscription page and select the free 2,000,000 character/month subscription package.
4- If you go back to your account page under "My data" you should see the following:
Create an Azure application
Once you have an Azure marketplace account with a subscription to Bing translator, the last step is to create an application. By doing this, you'll have a client key and secret you can use to call the API.
1- Head on over to the application registration page. Once there, fill out all the necessary information. For Bing translator, you won't be using a redirect uri (even though it's mandatory). You can put any valid URI you want to complete the form. ** Before hitting submit, copy the Client secret and Client ID. We'll need these later.
2- If you go back to the application registration page, you should see your app show up:
Call the API
Using the client key and secret from above, you have all you need to call the API. If you're going to be implementing a client to call the API yourself, you'll need to first authenticate in order to get an access token, then use that in the header of your requests for authorization.
I wrote some sample code(here) that retrieves the access token, before realizing there was a really great client for the translator written already called microsoft-translator-java-api. Here is some sample java code using this library which takes care of authentication and translation requests under the covers:
Here are some additional resources you might find useful:
Here is a more involved project that involves Twitters streaming api and bring translator:
Bonne Chance! Que la force soit avec vous.