With all due respect, I don't think (well, hope) things are like that anymore. At the very least JavaScript is something of a lingua franca that .NET developers probably also know. So at least that's something to fall back on.

If things seriously get as bad as you say, I would absolutely lie about the language I worked in / my experience. I won't feel good about it, but it's not too difficult to cram a new language/framework in a month or two and appear competent.

I would hope that people in charge of hiring aren't foolish enough to make that necessary.