belum berbahasa (dengan) lancar/ lancar?
How do I say "I do not speak Bahasa (Indonesia) fluently yet." ?

Is it correct to say "Aku belum bisa berbahasa lancar?" or do I have to insert the word "dengan" in front of "lancar"?

Actually, I have heard phrases with "dengan + adjective" many times, but I do not really understand the logic/ grammar rule behind them, so I always avoided using it.

I would be very grateful if somebody could kill two birds with one stone.

Question

belum berbahasa (dengan) lancar/ lancar?
How do I say "I do not speak Bahasa (Indonesia) fluently yet." ?

Is it correct to say "Aku belum bisa berbahasa lancar?" or do I have to insert the word "dengan" in front of "lancar"?

Actually, I have heard phrases with "dengan + adjective" many times, but I do not really understand the logic/ grammar rule behind them, so I always avoided using it.

I would be very grateful if somebody could kill two birds with one stone.

Answer

"Saya belum bisa berbicara bahasa Indonesia dengan lancar" = technically correct in terms of grammar, but I caution you against using this version as it sounds extremely awkward to the average Indonesian speaker.

"Saya belum lancar berbahasa Indonesia" = the best formal version you could use.

"Bahasa Indonesia saya belum lancar".= the least awkward-sounding sentence that you can use without compromising grammatical correctness.

Any native Indonesian speaker would agree that the third version above is the best-sounding sentence...as the former two would only make you sound like a PSA audio.