In case you are wondering why the book Write Your Own Programs uses T3X as its teaching language, here is why:
T3X is simple and easy to learn. The book teaches how to make a computer do things. A programming language is merely a tool in this process, so the text tries to get the technicalities out of the way as quickly as possible. Most popular programming languages are vastly more complex than T3X, so if one of those languages was used in the book, it would easily be 800 pages instead of 400.
T3X is a minimalistic language. The principles taught in the book transfer easily to other programming languages. Once you have mastered T3X – which will not take a long time – comprehending more complex languages will be much easier.
That being said, T3X is not a toy language. The author has written lots of programs in T3X in the past decades. Many programs in the later parts of the book are far from trivial.