Assuming he had a green light on the project itself, and it creates value, I don't see why that would be selfish.
Do what gets the job done, within reasonable limits. If the expected maintenance is minimal and you can improve your personal capability and/or produce the deliverable faster using a different technology, by all means go for it. I've had to learn languages far more unpleasant than Ruby (VBA, I'm looking right at you!!) in order to maintain programs that no one expected would ever change. That's life!
It's selfish if the numbers are such that maintenance ends up costing more than you saved by using your favorite language for the initial implementation. That depends on a lot of things: your co-workers' experience, your coding style, documentation for the language/libraries used that your co-workers aren't familiar with, etc.
Hint: any kind of problem that need only be solved once (or where it is cheaper to write the code again rather than to make it general enough for everybody) can be written however the original developer believes he will create the most value