As with everyone's great ideas and suggestions heres how I see it.
If your trying to keep your BoS secret or hidden, so that no one else can read it, sure you can use Theban or any other cipher, magical language or alaphebet and so on.
If your intent is putting your personal power and energy into your BoS then I suggest writing or coming up with your own magical language or alphabet, or find one that you personally connect to the strongest.
I have many BoS most of them are in English, however I have several that are written in several different scripts; such as Theban, Latin, Hieroglyphs, and several of my own magical scripts that I have created including my own runes and pictographs, and heck I have even formed an alphabet using various Reiki symbols. I am very creative with my BoS as well and I encourage everyone else to be as well. I have several fancier BoS that I have made myself out of parchment paper and binding, I have several mole skin lined journals I use, I have several Paper Blanks lined Journals with various elegant designs, and for easy orginization and managment I have a couple volumes of large three ringed binders with the plastic paper protecter sheets so that I can rearrange and organize my info, rituals, recipes, spells, and so on.
Anyway I hope I have given you some creative ideas, and sorry for going on a bit, but it is something that I am passionate about. :)