Close all OpenOffice applications
Obtain the file for your language from OpenOffice Dictionaries Wiki
Unpack the zip-file to the dictionaries directory, e.g.:
cd /usr/lib/openoffice/share/dict/ooo
unzip /path/where/downloaded/de_CH-20071211.zip
Run install-dict:
/usr/lib/openoffice/install-dict
Now you should see the new language module with Tools / Options / Language Settings / Writing Aids under "Available language modules"
Paths may vary, on my gentoo machine, the OpenOffice files are under /usr/lib/openoffice/]]>