It's easy to forget the value of delivering functionality immediately. It's all too easy to spend a long time on an elegant design that doesn't actually serve business needs. I like to follow an agile approach where if something can't deliver value in 2 weeks then it's not worth doing (or, more likely, needs to be split into smaller pieces). That's plenty of time to learn enough Scala to be more productive than you were in Java, even though it will take months or years to learn the full language.
Learning a new programming language is an intellectual, exploratory endeavor, even if it might result in adopting that language for future programming projects. Writing software professionally is a pragmatic engineering job constrained by non-intellectual considerations such as customer expectations (and lack of knowledge thereof) and limited availability of resources (time, money, skills, etc.). Suggesting that both activities should be approached the same way is a huge mistake.
I learnt scala very effectively simply by delivering a program in it at my day job. I'm not sure about "should", but in my personal experience I've found that the best way to learn.
