Although, keep in mind that a startup is going to be a roller coaster ride for years. You'll have days when you get tired of writing financial models for investors, or when an angry email from a customer ruins your mood, or you hit a road-block with a UI that just doesn't feel right. You'll want to take a break from all that by going back to just writing code for a day or two. At that point, you will want to get pleasure out coding. I'll be very unhappy if after those kind of days, I try to find solace in coding, and I see PHP, JavaScript or Java staring back at me.
Compared to the years it takes to turn a startup successful, the time needed to learn a new language well enough is negligible. I knew just basic Ruby when I switched to Ruby + Sinatra from Java for my new startup. Now, three years later, I can't be happier I made the switch.