Franker does require API key for Google Translate option only.
Microsoft Translator option can be used without API key (just put nothing to the respective field). In this case Franker uses hard-coded default API key.
In order to use Google Translate, you have to fetch API key from Google.
You can do it here: http://code.google.com/apis/language/translate/v2/pricing.html
This is paid service, you pay by usage.
You may want use your own API key for Microsoft Translator.
For example, to be sure other users service usage doesn't affect you (think about possibility Microsoft applies usage quota per API key in the future).
You can do it here: http://www.bing.com/developers/appids.aspx
The service is free (so far).
All above applies to both (Safari and Chrome) browser extensions and iOS app.