Bootstrapping Scala is basically impossible. You will not be able to get around the fact that to compile Scala 2.12.0, you will need Scala 2.12.0-RC2, to compile Scala 2.12.0-RC2 you will need Scala 2.12.0-RC1-1e81a09, to compile Scala 2.12.0-RC1-1e81a09 you will need Scala 2.12.0-RC1-ceaf419, you can keep going back this way in theory until you get to Scala 1 from 15 years ago, which is written in PiCo, a language which is basically Java-with-pattern-matching and whose source code was never released, so at that point you’ll need to write your own compiler.
I don’t know how Scala 2.11.8 got in Debian, but it’s impossible that it got it in through a full bootstrapping process, they must have cheated, so you’ll have to cheat again to upgrade it.