“Bahasa Indonesia” translates into “Indonesian language”, in which “bahasa” means “language”, and “Indonesia” means “Indonesian”.
Bahasa Indonesia = Indonesian language (or Indonesian)
Bahasa = language
Indonesia = Indonesian
If you say, “I can speak bahasa”, you’re telling the addressee that you can speak a language. Of course, humans speak languages. But which one do you mean?
It’s better to say, “I can speak Indonesian”.
I can understand if such error is made by a beginner language learner, but once you know the rule, don’t stick to the wrong version.
It’s not cool. It’s annoying.