I recently had a problem getting Librewriter to Automatically Spell Check my text.
I found the LibreOffice Automatic Check Spelling help page (see link) but this didn’t help because I had already checked the spell check option (Tools> Automatic Spell Checking)… but no wavy lines appeared.
https://help.libreoffice.org/latest/en-US/text/swriter/guide/auto_spellcheck.html?DbPAR=WRITER
After some head scratching I found I also needed to go to Tools>Language> and then choose between For Selection, For Paragraph or For All Text and pick a desired language.
I am thinking it might be helpful to include the Tools>Language>… options-settings in the spell check help page?