You're thinking along the right lines, but idioma is actually masculine. There are a lot of nouns in Spanish that break the o=masculine and a=feminine rule, usually ones that are borrowed from Greek in the case of masculine nouns ending in -a, or shortened forms in the case of feminine nouns ending in -o:

Thank you for the response.  But there is another flashcard that says una idioma.  Should that be un idioma?

Yes, it should be un idioma! Thanks for noticing that. We'll have to update the content. (The audio just says "idioma," so that's of course fine.)

Muchas gracias.

Muchos gracias.  Non problema.  La testigo esta en la moto.  (Why the witness is on the motorcycle, I don't know, but gotta practice the new words haha!)

In addition sadly not thaught in american books is the full list of letter endings in a word that says if a word is maculine or feminime.
The endings for maculine word is n.o.r.ma.l
Un jardín
Un niño
Un ordenador
Un dilema (ending of ma are from a greek origin and it's like that in all latin languages)
The word alma is from a spanish origin and it will be feminine and it means soul.
Un árbol

The endings of feminine words a, ción/sión,tad,dad,tud,z

E for professions and cualities are unisex
And in regular word it can be or femenine or masculine and remember by hearth.
S words end can be likeways
And words which and in ista are unisex

The ending can help to identify the gender, but it is not always conclusive:

Common masculine endings
	Examples	Common Exceptions
-o	el teléfono, el foco, el ruido	la foto, la mano, la moto
-r	el rumor, el temor, el lunar	la labor
-n	el examen, el régimen, el don	la imagen, la razón
-l	el ferrocarril, el sol, el árbol	la catedral, la cárcel, la piel, la sal