This problem still exists in 2015 in Linux Mint 17.1. To remedy the situation, you will need to install
myspell modules for your preferred language. E.g.
sudo apt-get install myspell-en-us
Then after doing this, comes the KEY step.
Open LibreOffice and go to Tools → Options → Language Settings → Writing Aids and select the spell check modules appearing there and
Edit them and change the language to your preferred language which has a spell-check module installed.
Also, set your preferred language in Tools → Options → Language Settings → Languages. Restart your libreoffice for the settings to take effect.
In both these setting you will see that the languages which have the spell-check modules installed will have a little spell-check icon besides their name, as shown in the screen-shot.
Screen-shot for first setting
Screen-shot for second setting