In Spanish, "lengua" also can mean the actual tongue in your mouth.
In English, referring to a language as a "tongue" is somewhat archaic. "What is your native tongue?" = "What is your native language". In Spanish, however, it is sufficiently common that "lengua" and "idioma" are practically synonyms.
Here's a link about idioma vs lengua vs lenguaje: https://forum.wordreference.com/threads/idioma-lengua-lenguaje.7440/
Marcy, I understand that articles are used when talking about something in general however I noticed you also specified that spanish is the subject in this sentence (which is true of course). I'd like to know if that (being the subject) is also a requirement for the 'general' rule regarding articles or does it also apply when the noun in question is the object of the sentence?
It's not precisely a requirement, but general statements* are only made about subjects in the vast majority of cases. Objects* are usually more... precise? Non-general. Objects* are mostly defined, small-portion things.
* These are generalised subjects, by the way.
But in something like "Horses like apples", you can have both items generalised. "A los caballos les gustan las manzanas."