Firstly, Justin, congrats on the new version, looks exciting!

My question, of course, is if there is a big difference in the language files. To upgrade to the new version in Spanish, do I need to translate the whole lot again?    

I normally put spaces between the ones you need to translate between versions.  Just translate the ones after those lines.  It'll still be a lot to translate though.

OK, well that´s a great help, as we have volunteers that will do it wilingly! Just to check, as an example, in the language file:
calendar.lng you have the lines:

-----------------------------------
$calendar[108] = 'month on the same day.';
$calendar[109] = 'month on the same weekday (Sun-Sat).';
$calendar[110] = 'year on the same day.';
$calendar[111] = 'year on the same weekday (Sun-Sat).';
(blank space)
$calendar[112] = 'You are over the calendar event limit.';
$calendar[113] = 'Upcoming Events Feed';
(blank space)
$calendar[114] = 'Search Calendar Events';
$calendar[115] = 'Description';
$calendar[116] = 'Personal Event?';
$calendar[117] = 'Between Months (MM)';
$calendar[118] = 'Between Years (YYYY)';
-----------------------------------------

As I understand it, the new lines to translate would be:
$calendar[112] = 'You are over the calendar event limit.';
$calendar[114] = 'Search Calendar Events';

(the lines after the blank spaces) Is this correct?

As long as they weren't translated in the old version.

IE, if the old version left off with $calendar[10], then start translating from $calendar[11] until the end of the file.

Ahh... got it now. Thanks and I´ll get cracking!