(I'm the author) Just for the record, although it's hard to exactly quantify the extra amount earned from the localization, even a conservative estimate covers our localization costs and a small profit. You're right in pointing out that opportunity cost is the real cost.

If this was the only game I was ever going to make, I probably would have spent my time better on things other than localizing it nearly a year into its lifespan.

I think the most important point is that we left plenty of money on the table by not having the localization available on day one. The localization was launched very late into the game, and almost all revenue comes from the major promotional periods.

It will be interesting to compare results with the sequel to see if we have notably larger market share right from the start in the languages we choose to localize in, I think that will provide a little more conclusive data than this admittedly murky analysis.