If you say that you don't know C++, but you know SOME language which you can discuss in a meaningful manner, that's fine too.
Regardless of the job you are applying for, I expect you to be a quick thinker and demonstrate good problem solving skills. Beyond that, if you are applying for a senior position, you better know SOME language very well.
And anyone who says that C#, Java, Python or some other toy language is "better" must never have written software where performance actually mattered. If you are writing GUIs for a phone, then use whatever language gets the job done best, but when you are writing software that has to get massive amounts of work done, on limited resources, and needs uptimes that are measured in years, then you use a language with less run-time overhead and more predictable results.