Franker requires API key for Google Translate option only.
Microsoft Translator option (default) 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. 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). The service is free (so far).

All above applies to both (Safari and Chrome) browser extensions and iOS app.