$ sudo apt-get update
$ sudo apt-get install myspell-de-ch 

myspell-de-ch is installed in your system.

Make ensure the myspell-de-ch package were installed using the commands given below,

$ sudo dpkg-query -l | grep myspell-de-ch *

You will get with myspell-de-ch package name, version, architecture and description in a table

myspell-de-ch package basic information: