Definitely. If your main goal is joining a startup with a chance of success then you should be evaluating the people involved. Their language choice should be a data point, but ought to be taken strongly in context of their backgrounds and the problem they are trying to solve.

In the context of I want to accomplish as much as possible in my life, all other things being equal the particular language you happen to be using for the next few month-years will probably prove insignificant.