It’s possible to add another language for spellcheck by editing
C:\Users\UsErNaMe\AppData\Roaming\Logseq\Preferences
(Windows example)
If you do this both English and German are spell checked, even if it’s written on the same page.
Example, added German:
"spellcheck":{"dictionaries":["en-GB","de"]
To make Logseq more accessible and beginner friendly for everyone it would be nice to have a settings option that lets you define which languages should be spell checked.