I'm not sure that it's entirely that simple. I don't want to get into any language wars but certainly in some cases the one single developer coding in a language they are extremely proficient in may in fact help use the "runway" most effectively.

The cost of collaboration is not equal to 0, and if it was my money I'd be looking for optimal efficiency over broad accessibility.

My vote would be for option 1, as you have already indicated that this affects only 40% of your product. If you do experience continued steady growth and become cash positive then pull in extra resources to support the 40%.

Given of course, whatever the 40% is written in is considered to be a good language. I won't define good, it means different things to so many people...