Allow custom translation

This is awesome!!!!!.......... Mind switching those tabs to spaces?

This looks like a good start, but it looks like you'll only allow one set of alternative strings. Users should be able to set these for each language. I suggest saving one array as a variable, indexed by language code, whose values are arrays of these strings.

Also, it would be better to leave these fields blank, so that users can choose to override a subset of these fields. The default text could be written underneath. In combination with #356348: i18n language support this would allow users to easily override just some strings, while having others supplied by the folks at reCAPTCHA, as appropriate to the current language.