You don't lose experience when switching languages. The degree to which your experience applies will depend on the degree of difference/similarity between the two languages though.
All programming comes in (at least) two parts: one is a language-independent ability to analyze and solve problems in ways that can be implemented with a computer.
The other is turning a method that's suitable for implementation on a computer into actual code that will run and do what the customer wants.
The first obviously isn't affected by moving from one language to another. To some degree or other, the second is -- and even if you continue to use the same language, changing things like libraries or application frameworks has at least some of the same effect.
Unfortunately, when it comes to real-life experience, it's difficult (if even possible) to separate the two that cleanly though. Your ability to solve problems in influenced (to at least some degree) by your ability to visualize a solution, which (in turn) requires thinking in terms of at least some abstraction of a target language (i.e., not necessarily in a specific language, but at least with a fairly decent idea of what sorts of things can be expressed/done in a programming language).
There's also the fact that different projects require differing amounts of the two skill sets. Some (e.g., a lot of web sites) require almost no analysis or problem solving, and are nearly pure coding. Others may require extremely intricate problem solving, but at the end of it all, relatively little code -- and just about anything in between.
You'd at least hope the employers would base their evaluation of your skills on the types of problems they solve, but I suspect that few have ever even tried to analyze their market in these terms, so most don't even have a basis for doing so, not to mention the desire or HR people who'd be able to even if they wanted.
Edit: Looking at a few other answers, I have to dispute the general implication many of them carry. Yes, solving problems is part of the job, but making good use of a target language, framework, etc., is a lot more than just syntax or anything like that. The target affects not only the syntax, but the "shape" of the solution as well.