Relation between semi decidable, undecidable and countable sets
Undecidable is simply the complement of decidable, as the name suggests: anything that is not decidable is undecidable. So the whole pink area of your diagram consists of undecidable languages.

All languages over finite alphabets are countable. For example, every string over alphabet $\{0,1\}$ is a natural number written in binary.1 Everything in your diagram is countable. We only ever consider finite alphabets; infinite alphabets aren't physically realizable because we can't store infinitely many distinguishable possibilities in a finite space.



1. A small problem is that $0$, $00$, $000$, etc. are all the number zero written in binary. So, instead, associate the binary string $xyz$ with the natural number $1xyz$ and now, every binary string represents a different natural number.