Open up your Terminal and type
sudo apt-get install libmythes-dev
then
cd /usr/share/mythes/
sudo ln -s th_en_US_v2.dat th_en_GB_v2.dat
sudo ln -s th_en_US_v2.idx th_en_GB_v2.idx
Restart OpenOffice.
Problem solved. (I hope so :D It works for me and I hope it works on you too.)