I installed Collabora directly from the deb packages and realized that the spellchecker dictionaries were for English only.
But it is easy to enable them for other languages too merely by copying the respective files over the respective files from a desktop Libre Office
scp … cp … wherever it is installed:
scp -r mydesktopserver:/opt/libreoffice6.0/share/extensions/dict-de/ /opt/collaboraoffice6.0/share/extensions
Don’t even have to restart.
Strangely enough in the debian packages all languages except English seem to be missing though they pretent to “speak” at least en fr es ru de …