I had similar problems for checking my spelling in German. I had to download additional packages to fix it. I used this route:
Tools > Options > Language settings > Writing aids
Click on the edit button next to “availible language modules”
Click on the link for “get more dictionaries online”
Search for the dictionary you require, download and open the file. Restart and open libreoffice.
Should now recognise the language and spelling errors automatically - although I already changed my operating system locale and preffered languages to german so you may have to select your preferred writing language in libreoffice as an extra step.