I am doing infrastructure for a well known, yet not public SF company. I've had days where I had to write code in 5 languages, going from interpreted languages with almost no type support, to Scala's type battleship. This raises problems for learning: I learned Scala somewhere else, doing nothing but Scala for months. When you can dive deep into a language, it's easy to remember it, and then bring back the knowledge on command. But without a deep dive, it's really hard to learn a language, especially when you deal with a bunch of similar ones. I never have to do much Ruby, or much Python, so they never stick in my head, and the lack of a compiler means I often write terrible, broken things. Maybe if I only wrote ruby for 3 months, then only python for 3 months, then maybe they would stick, but that's not now infrastructure work goes.