How? Because in their idiocy, TT offers a map too large for the storage space (not memory) on your unit.
I gather the map was downloaded. Use Finder to move to the location of the downloaded map file. On a pc, the location would be:
C:\Documents and Settings\yourname\My Documents\TomTom\HOME\Downloads\complete\maps\
I don't know what the file structure is on a Mac but if you check using Home, Tools, Folder Locations, you'll see where it shows downloaded files for you.
Once you've found the compressed cab folder, you need Stuffit to look at the contents. Within the voice folder, you'll see a bunch of files looking like dataxx.vif and dataXX.chk where XX is a number. The vif file shows the language of the voice when you open it in a pc with notepad, so use Mac's equivalent to notepad to look in the .vif file. Every .vif file NOT in English, remove that from the cab file AND the associated .chk file.
Once all the non English .vif and .chk files are removed from the cab file, the resultant (smaller) cab file should install.