Again, the "cannot" here is probably imprecise. In Japanese, r and l exist as a single sound, making differentiation difficult for the average Japanese speaker

Again, the "cannot" here is probably imprecise. In Japanese, r and l exist as a single sound, making differentiation difficult for the average Japanese speaker in other languages. The original research by a Japanese linguist into this phenomenon in the 70s suggested it was impossible because of the way phoneme and allophone acquisition happens in childhood. But this claim had been mitigated by subsequent research that suggests it is context dependent, as well as less certainty about phoneme vs. allophone distinctions.