How are credits calculated?
1 credit == 1 character of text to translate. In average translating to one locale will cost about 5000 credits, however small apps will afford a lot more.
Is this SDK free?
You receive 10 000 credits for free when you log in with your Google Account. For more you need to pay in Store available in the app.
What languages should I start with?
Start with top languages list from Google Play: