Hard for me to vote for a single language here. Recently I've been flipping between Go, C and Lua in fairly equal measures. My 'primary' language only exists for some short sample period as by necessity I'm flipping between languages as needed by projects.
The customers from our consulting projects dictate what languages get used, not me. So each new project usually means complete context change in terms of OS and Languages, in regard to the previous one.
Additionally all projects tend to be polyglot to a certain extent.
