I have programmed in a proprietary language. But that was not the language that was used all over the place. I have nothing against that experience now, though I whinged every moment of doing that work. I did get chance of using that language, maintaining that language and, improving it, along with maintaining the program that were written in that language.
Along the way, I learned to read and understand how a small interpreter would work. If your potential employer has the sources for the compiler/interpreter for the proprietary language, you might just as well get a chance to work on its internals. I can say, from experience, you will treasure this work experience for years to come.
All this, assuming that your potential employer has the sources for the proprietary language and you are willing to more than work on the programs written in the proprietary language. Still, I admit I am being extremely optimistic in saying that you might get a similar opportunity like I got a few years ago.
At interviews, years of experience and technical skills are not alone in hiring decisions. Your thirst for learning, cultural fit, cultural leaning, make a big part of the decision.
So, for a first job, if your work will not be limited to the programs written in the proprietary language alone, but allow you to work on its implementation as well, take it.