I would definitely mention the experience with the language in the description of the job, stating that you did something with a proprietary scripting language. Having experience with such a language would give you a leg up on learning other languages that are similar. It also tells perspective employers that you have the ability to learn new languages on the fly, an important skill. Plus, it's something you can talk about in a technical interview.
However, how you present it will make a difference. Like JB King said, don't present it as you writing an interpreter or designing the language (unless you did those things). Instead, mention that at your job, you were presented with a proprietary scripting language and perhaps mention a couple of things that you used it for. At an interview, if the company you are talking to uses a similar language, you can be sure to bring up how you might not have experience with that particular language, but you used a similar language and then (if you are able to), discuss the language you used.
If you have a "skills" section of your resume, I wouldn't put it there. Instead, tailor your skills section to highlight what skills you have that the job you are applying for. Or you can drop the skills section all together and use your job descriptions to highlight the various skills that you have in some kind of context.