If you're great at
C++, I suspect that filling in some hole in another part of your skills portfolio is optimal.
Forgive me for saying this, but the odds are that the best IT investment of your time is probably more
C++, not another language.
Unless you are really really good at
C++, the sort of jobs you probably want are more likely gained by a bit more
C++ than half-finished skills in another language.
A quant who absolutely kills every
C++ question thrown at him will do better than one who gets 99% at
C++ and 80% of SQL/Java/VBA/Matlab
Occasionally SQL seems to be in interviews, not often though.
But in balance, in a tough market, there is another way of looking at your skills.
A few managers like minority languages. Someone with F#, SmallTalk, Fortran, REXX, won't hurt his chances with most people who filter CVs. But it
might catch the eye of a recruiter who then pulls you in for interview.
It's a free option, albeit one deeply out of the money.
OK, not "free" because you have to be able to answer basic questions on it, if you put "basic F#" he
may see you as a person like him who thinks out of the box.