A caution: if they're doing everything else right except the language choice, then chances are they will end profitable and successful by at least some definition of success, but will not be as successful as they could be. It's very tempting in this situation to sit back and enjoy being an employee at a successful company. That will not get you any of the big payoffs or excitement of a startup.

If you're going to settle on programming language choice so you can learn all the other stuff, you have to be willing to cut said successful startup loose and strike out on your own once you've learned all the other stuff. This will look increasingly irrational the better you do at your job.