This just depends on the company. If the technologies requested are at least related to work you have done in the past, I would play up my domain knowledge and ability to learn new languages quickly (which it would appear you are already doing).
However, some companies are looking for someone that can fill an immediate need. They want someone who has knowledge and experience in the specific technologies they are using such that they require little or no training and can be productive on day one. These sorts of companies tend to stick to widely adopted technologies (Java, etc.) so that they can find replacements quickly. These companies will generally prefer someone of average talent who has X years experience with the right technologies over a genius with experience in other areas. If you don't have the skills, you won't even get an interview. No great loss, you probably wouldn't want to work at such a place anyway.
Other companies (usually the smaller ones, but not always) tend to seek out talented software engineers first and consider experience with the right technologies to be a bonus. Usually you'll be able to tell which sort of employer you're dealing with early on in the application process (if the job posting itself doesn't give it away).
If they don't care what language you use to solve problems during the interview process (either pre-screening homework or 'whiteboard' questions during the interview itself), that's a good sign. I have been known to satisfactorily complete such an 'interview homework' assignment in an unfamiliar language only to be rejected later on when the interviewer(s) learned that my experience in language X was basically limited to said homework assignment :/
Alas, I have had to develop better filters on my side of the interview process, and a lot of it comes down to gut feelings (based on experience) that lead me to say "Sorry, no longer interested" :)