From The Document Foundation Wiki
- A dictionary must be installed for your language. The installation is done by an extension as explained in this item. You must close LibreOffice completely, including the quick starter, in order for the new dictionary to be taken into account. The default dictionary for your language is installed.
- Check that the dictionary is installed:
- Go to the ▸ ▸ ▸ ▸ menu, click the Edit button.