Thanks for asking! The Arduino programming language is C++. Here’s some sidebar text from Chapter 2 in the book, which should help clarify:
People sometimes seem to be a bit irritated when it comes to
the language the Arduino gets programmed in. That’s mainly
because the typical sample sketches look as if they were written
in a language that has been exclusively designed for programming
the Arduino. But that’s not the case— it is plain old
C++ (which implies that it supports C, too).
Every Arduino uses an AVR microcontroller designed by a company
named Atmel. (Atmel says that the name AVR does not
stand for anything) These microcontrollers are very popular and
many hardware projects use them. One of the reasons for their
popularity is the excellent tool chain that comes with them. It is
based on the GNU C++ compiler tools and has been optimized
for generating code for AVR microcontrollers.
That means you feed C++ code to the compiler that is not
translated into machine code for your computer but for an AVR
microcontroller. This technique is called cross-compiling and is
way to program embedded devices.
Susannah Davidson Pfalzer