Explicit:
"I have an Arduino-compatible board, that I program in the Arduino templating language, using the Arduino IDE".
How aout "I have an Arduino-compatible board that I program using the Arduino IDE" ?
The reason that you don't see the language explicitly called out is that hardly anyone cares.
If you must, it would be: "I have an Arduino-compatible board that I program in C++ using the Arduino IDE"
It would make it clear that it is a templating language
Everyone keeps telling you that it's C++, and any other wording is worse. You say our answers are "very helpful", but you remain stubbornly unconvinced of that basic fact, and are making your description worse and more obscure the harder you try.
I give up.