Took me a while to figure it out too, I ended up adding a language addition so my client could change it.
Seems to be an oversight by x-cart team that's been in the system for ages, even 3.4.2 still has that problem. Not sure why they simply haven't added an new language variable in there.
Sometimes you get the feeling they have too many developers working on small bits of x-cart all the time so no one is looking after smaller issues like this one.
Personally if I didn't know PHP/mysql etc I wouldn't use x-cart, it's a large complex system and doesn't always make sense about how it goes about some things.