As with any kind of marketing, the best way to go about it is find out what appeals to your customer base and formulate your message and advertising based on that.
IMO, most non-technical website owners don't give a crap about what language you used. They want something pretty, and something functional. They want an interface that is simple and easy to use. Focus your message on how the end-product will look and behave, and provide example websites demonstrating this.
With all of that said, I have no idea how anyone makes a living doing freelance website work, especially us folks living in North America. We are undersold by cheap labor (by cheap I mean inexpensive as well as "cheap") from overseas which makes it difficult to compete. Most people are going to place higher value on the price point than they are the quality of the end product. I've seen lots of garbage code from India but if the website looks OK, most normal people don't give a crap how easy or hard the code is to modify later. Until, of course, it becomes later, and they need their garbage code modified. Then they can't understand why they're being asked to pay so much for a "simple" change.
But try convincing someone they should pay twice as much to get the website built, just so the code can be easily changed 5 years down the road.