Having the on/off, is not as easy as it sounds.
You need to do several thing for it to "work" properly.
Use navtweak.
Menu text.
1: export the text language file for 1st language (rename it to the 2nd language filename).
2: edit the 2nd language file as the 1st (remember to keep the top 2 lines of the original file in place).
3: mod the text language file "language" for the on/off text, in both 1st and 2nd.
4: change the language string to to "poi alert" (or similar), in both 1st and 2nd).
5: import both 1st and 2nd files.
6: change the first language id (USA,GB,D,F, whatever) to "on".
7: change the second language id (USA,GB,D,F, whatever) to "off".
8: replace the 3rd language option with a blank.
Voice files, for alert.
9: copy the 1st voice file to the 2nd voice files name.
10: change the 1st voice file so that it has the alert.
11: create a new sounds dir (like POISon says) 0507 and copy all the files from 0506 to them including your modded 1st and 2nd voice files.
12: edit (or use) the modded files so that the new sound dir is used.
Map disc Mods.
13: mod the 2nd language files .LSC file in the map disc so that the "off" option has the same mods that your "on" option does. (if you have GB and USA then it will use the same ENG on the map disc).
Example:
HTH