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.
Duolingo clearly translates 'lengua' as 'language' so I don't think this fully answers Unepersona37's question, unless you're implying that 'lengua' should be used in the same context as 'tongue' in English when discussing language (i.e. 'mother tongue', 'native tongue'). If that was your intention, I don't think it works out here, because I would rarely say 'Spanish is an interesting tongue'.
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."