My language settings are set to German but changing the spelling settings does not work. What is the reason that within the settings menu, there is no option to select German. If the reason is that I need to install a different dictionary, why is there no option? How do I do this?
The applicationi Manjaro Settings Manager offers to install language packs based on the chosen system locale.
If you need other spelling packages - the system offers to install additional languages - which then will trigger the option to install additional spell check libraries.
You can of course install those manually using pamac - but you will have to look them up manually; it may also be necessary to add different dictionaries based on application and type of dictionary.
As I reckon your language is German you could grep the query for the country code or language identifer e.g.
pamac search dictionary | grep -e 'german'
Or
pamac search spell | grep -e 'german'
The same can be done in the Add / Remove Software application. Just remember to filter out the AUR by selecting the Repositories option
Also be aware that Plasma uses the file ~/.config/plasma-localerc which may overrule what is defined on the system level (/etc/locale.conf and /etc/locale.gen)
These are Plasma settings. However Plasma doesn’t really have a spell checker of its own. That is, the actual spell checking across K-apps like Kwrite or Kate or the spell ... in Krunner is performed by some 3rd party spell checker. There are different ones. I have hunspell installed. Then the spell checker program needs dictionaries like hunspell-de and so on for the languages you want to use. You can install these packages using the pamac gui and searching for hunspell. After installation you also need to restart Krunner from the terminal as your normal user (no sudo) like
systemctl --user restart plasma-krunner
There might be further hiccups depending on the language selection and the settings like Enable autodetection and Automatic spell checking enabled by default. Non-K apps may come with their own spell checking and dictionaries - like LibreOffice I think.