Is the spell check in your copy of LibreOffice not working?
When I installed Ubuntu 17.10 and set my locale to English (UK) during the install LibreOffice correctly noted the locale, but didn’t pick up the English (UK) dictionaries, meaning that spell checking wasn’t working.
Luckily it’s an easy fix:
- Download the latest dictionaries extension from the LibreOffice site (the UK English ones are here: https://extensions.libreoffice.org/extensions/english-dictionaries/)
- Then in LibreOffice hit up Tools -> Extension Manager and click the ‘Add’ button
- In the resulting file dialog box find the .oxt file that you just downloaded and double click it
- Restart LibreOffice
Voilà! (if you type that in LibreOffice Writer it should now have a red squiggly line underneath!)