To add a new spelling dictionary, hyphenation dictionary or thesaurus files for another language, at least for OpenOffice 2.x:

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/