yes you can!
you need use XS++ or FAR and main2raw and SEFStool4 to do it.
first is to find the language that you want to add to phone is in which FS file. for example if you want to add German to your phone then you need W800_R1NA005_FS_EUROPE_3_RED36 or which ever the FS that have your language in it to check that you can go HERE to find out.
Use the main2raw and SEFStool4 to extract the FS into folders
The files you want are under:
\TPA\PRESET\SYSTEM\LANGUAGE\DE.LNG (DE is German. Language Acronyms also be found at the bottom of the Languages Description page)
Copy the DE.LNG into corresponding folder on the phone and delete LNG.dat and LNG.dst on your phone in the LANGUAGE folder under \TPA\PRESET\SYSTEM\LANGUAGE\
EDIT the Customize.xml file
\TPA\PRESET\CUSTOM\Customize.xml
<allowed-language>de</allowed-language>
in the correct section and save the file as customize_upgrade.xml this is a must, can't be another name.
and also edit the allowed_language.txt file
\TPA\PRESET\SYSTEM\LANGUAGE\allowed_language.txt
de,
save it as allowed_language.txt the name stay the same.
and upload these files to your phone as well
restart you phone and you will see the language you just added.
for adding Chinese language to phone you do need to flash your phone with the MAIN HONG KONG, CHINA OR TAIWAN, if not then you only can read the Chinese on the phone but can't not write. other then that the steps is the same.
Hope this helps.