UN "IDIOMA" NO ES UN "IDIOM"
There is a lot of confusion about this because although these two words are very similar, they mean different things.
The Spanish word "idioma" means a "language", but I think that is quite clear for everyone. The word "idiom" in English is a little more confusing, however. You would expect it to mean a language as well but in fact, the real meaning of an "idiom" is "an expression" or "frase hecha".
"I heard it on the grapevine" is a good example of an idiom.
Hope that helps!
Soy el director de Daniel Smith + Partners. Tengo más de 15 años de experiencia enseñando inglés a españoles.