I'm a recent college graduate and have been hired to work for a business software giant. The job by itself is great with amazing perks and a decent pay. Also there is the joy of working in core product development, building something which will be used by millions of people. However a huge problem is that we do most of our development in a proprietary language which is not used in any other product development company outside of here. The only other companies using this language are our partners and customers who would like to implement/customize the software in their business/clients' businesses.
Im afraid that working here for a length of time could make me irrelevant as a developer outside the company. While ideally one could say that software development is software development and the language does not matter, the fact is that most companies hire based on past experience, and in this context, my experience would be 0. My apprehensions are supported by the fact that that the attrition rate in this company is much less than the others, so you could find a lot of people who have not changed their job in the past 10-15 years. This is great from the company's perspective and they take a lot of pride in this. However I'm sure that most people do not leave cause they are tied up, since their skills are irrelevant outside of the company.
I really love programming and want to remain a programmer in product development rather than being a manager. What should I do to be hireable in a new company if and when I choose to leave my current job? Thanks in advance.