LibreOffice Spell Checker doesn't work?

A quick fix for the current document (assuming English text) is Tools - Language - For All Text - English (USA).

I think the permanent fix is to make sure that the default language used for documents is supported by an installed dictionary. This solution has been documented by "idiota" on LibreOffice's questions forum:

"The problem is fixed by clicking Tools/Options/Language Settings/Languages. It allows dozens of flavours of English, BUT spelling is disabled for all the ones that don't have the little blue tick ABC icon beside them."

http://ask.libreoffice.org/question/1678/spell-checking-not-working-in-v342/

When choosing your language do you see the check besides it?

If not then open up "Language Support" and click on "Install / Remove Languages..." and install the language you want and close LibreOffice and try again.

To install the spell check packages for English and USA the terminal commands are:

sudo apt update
sudo apt install myspell-en-gb myspell-en-us

Notes:

• If you already have installed hunspell-en-gb then you have to remove it to install myspell-en-gb.
• If you already have installed hunspell-en-us then you have to remove it to install myspell-en-us.