Bug report contains information how to get it:

One item that wasn't working out of the box is spellcheck: Settings > Configure KDevelop > Editor > Editing > Spellcheck would display "No backend found for spell checking". After grabbing the debian sonnet-plugins package and extracting aspell.so, hspell.so, and hunspell.so to usr/lib/qt5/plugins/kf5/sonnet/ (in the tree extracted from the AppImage), spellchecking is back in business. Perhaps you can include these libs in future AppImage distributions.