The information posted in the following link about changing the spell check dictionary is accurate BUT...
Zimbra Desktop 2 FAQ - Zimbra :: Wiki
it fails to show an essential piece of information:
You have to change the ID number of the dictionary you are installing to equal that of the original dictionary installed in your ZD, and then remove the original dictionary files (ZD can only handle one dictionary at a time)
For example, if you want to use the Spanish dictionary, you'll find the dictionary has 2 files, one Notepad file "es-ES" and one .aff file "es-ES.aff", open the Notepad file of the original dictionary installed in your ZD, you'll find a five digit number right at the beginning of the text,
1) copy the five digit number from the original dictionary
2) paste the five digit number to the dictionary you want
3) remove the original dictionary, so you leave only one dictionary in the 'dictionaries' folder (because ZD can only handle one dictionary at a time).
4) restart ZD
Note: Macs users may have difficulty when trying to modify the Notepad file and can solve it by modifying that file on a PC and then pasting it back to the Mac.
Hope that works for you!