In my case, all of the above tips checked out as "working". Safe Mode showed the problem wasn't with my profile, the Language Options were set correctly.
It turns out that my specific problem was that I'd selected Canadian English as my default language, but sudo apt policy hunspell-en-*
indicated that only the US English dictionary was installed. sudo apt install hunspell-en-ca
fixed my problem. I restarted Libre Office Write after installing the dictionary just in case, I'm not sure if that step was needed or not.
This problem was particularly vexing because the spell check claimed to be working but never flagged any of the purposefully wrongly spelled test words so very clearly wasn't working. Just "Spell check complete" every time. As a programmer, functionality should never fail and be able to claim success.
I'm not sure how my system got into this situation, but I have a theory. I wiped and reinstalled from scratch Ubuntu 18.04 a few months ago. Then I restored various data from backup, including a number of spreadsheets and documents. I suspect that on my pre-wipe system, when I set Canadian English, it auto-installed the proper dictionary. When I opened an old document on my post-wipe system, the old settings in the document were used to update my new fresh profile, but the proper dictionary install step was not done.
Posted in case this helps someone else.
Have you checked in
Tools
->Options ...
,Language Settings
->Writing Aids
that the modules, dictionaries and desired options are enabled?Please provide more information for questions like this. Like: has it worked before but is this a sudden change in behavior of Libre, or have you just installed it and doesn't it work out of the box, like that. If you are completely new to LibreOffice, you don't need to reset the user profile as in my answer.
Mine did not work. I downloaded the extension for English (USA) which was part of the latest bundle and installed the bundle and I think I added English (USA) to my dictionaries. Still it did not work. I searched here and found all the options suggested in the other answers were checked already including hunspell. So then I changed from the default English USA to this one I had downloaded in the Options > Language Settings > Languages > Default Languages for Documents. Then after Libre Office restarted, it worked.
I had to make sure LibreOffice knew my text was English. Select all text, right click > Styles > Edit Style. Click the Font tab. Don't change the font, just change the language to "English (USA)"
You can make this automatic for new documents in the Options menu, as Stephen Mikesell notes above.
This Worked! My LibreOffice had been working great for years, then suddenly the spell checker just gave me blank forms. I did the "Writing Aids" but it didn't help me. The "Default Languages" was set to German. I don't know how that happened, but I speak English (American Southern Style!). That change fixed it. THANK YOU!