To update languages to any official languages eg EN US DE IT FR SP:
1. Get Navcoder and install it and run on PC
2. Connect PC to car with ibus interface
3. Select Code Navigation, and select the 2 x languages you want, and click Code Language
4. Insert OS CD into nav when prompted and wait for language upload, follow onscreen instructions

To hack an OS to make your own languages:
1. Get and install NavTweak
2. Decide what language you will change, eg EN to PL
3. Browse the OS CD image to \Mk3\0116\BMWC01S\app_sw, open usw_load , go to the Strings tab and extract the EN language text strings by clicking Export (tip:0116 = your hardware version v1.16, check with your nav that it is the right hardware version)
4. Translate all EN strings to PL strings at your leisure
5. Load the PL strings back into the usw_load file by using NavTweak Strings Import, and put the usw_load file back into the image
6. Rename the PL voice to the same name as the EN voice file (e_f.csf) (in langMk3\0501 directory)
7. Change the version files from 3.63 to eg 3.64 to ensure you force an update, and so that your new hacked version is different to standard version
8. Burn the OS CD (ensuring Mode2/Form1 at all times)
9. Insert OS Cd into nav and follow onscreen instructions

For more detail, search this forum and others for NavTweak

Or ask on PL BMW forums, maybe someone already has made a PL OS language.