Spell checker not working in Libre Office 5.2

I tried Brayden's answer but I also had to go to Tools > Language > For all Text > English (USA) to get it to work.

I had to install a dictionary to fix it. apt-get install myspell-en-gb worked here in the UK.

If you are using a more recent version of Ubuntu then you now need the hunspell-en-gb package, so

apt install hunspell-en-gb

I am on Ubuntu 18.04 using LibreOffice Writer Version: 6.0.3.2, I encountered the same problem, and to solve it I had to install the language package. I used sudo apt-get install hunspell-en-gb to install the English language pack, as sudo apt-get install myspell-en-gb, did not work for me. I hope this helps.