A couple of times, I've noticed Japanese people mistakenly use "ram" instead of "lamb". For example

You broil lamb [correct] or mutton with vegetables. There are roughly two ways to have Genghis Khan; either you broil the marinated meat or dip the broiled meat in sauce. The difference between mutton and ram is in stages of growth. The sheep for mutton is two or more years after its birth, and that for ram is less than 12 months after its birth. [emphasis added]

This made me wonder: is the similarity between the two words, apart from the l/r difference, describing the same animal a coincidence?

Etymology online says that ram and lamb are West Germanic and Germanic respectively, while Wiktionary traces lamb back to Proto-Indo-European but merely says that ram comes from Old English.

Is it known whether the two are related? Failing that, is a change from an "l" to "r", or vice versa, unlikely enough to rule out this possibility?