My guess is that you installed the regular code package from repository, as the docker instance has all spell check pre installed.
I bumped into this also:
Collabora Online 3.0 out with full-featured editing dialogs
Depending on what distro you run, you can install hunspell packages for the languages you need.
As a collabora developer notes:
Tools -> Language for .... is showing code instead of options
It apears that hunspell should be used instead of myspell