"El español es una lengua interesante."
Translation:Spanish is an interesting language.
Technically idioma = language and lengua = tongue. That said in either Spanish or English a language can also be referred to as a 'tongue' though I don't believe it is common in either case.
In portuguese we have these similar words too: ("idioma and língua") "Língua" is your tongue (part of the body) And "idioma" means your language I think this is valid for Spanish too
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'.
It's the subject of the sentence, and we're talking about Spanish in general, as a whole.
Life is beautiful. La vida es hermosa.
Cigarettes are dangerous. Los cigarrillos son peligrosos.
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?