There has been a lot of studies that have tried to teach recognizing the audible difference of R and L to Japanese English learners. I don't know of a single study has been able to produce robust results. (There's a primary and secondary acoustic cues to the difference; some Japanese have had success recognizing the difference using secondary acoustic cues, but recognizing the primary cue which is the most robust one and which the natives use preferentially escapes treatment.)
However, the testing protocols and time are limited, and we have increasingly knowledge about adult neuroplasticity. Results like this keep me hopeful that it's just that we don't know the good way to do that yet and we might see some results in the future.