I've completed coursera courses in different languages and referenced them in cover letters, but with no feedback I assume this makes absolutely no difference.
How did you manage to even get an interview for a position using a language/environment you're not familiar with? Am I unlucky having to deal with the stigma of "uncool" .NET?
Just get familiar with it. Post an interesting small project to your github written in that language. Blogs are often useful for demonstrating your interest and aptitude for technologies. All of that is attractive to an employer.