Ok, let's be clear: It's NOT a stereotype or an invented speech pattern. The VAST MAJORITY OF JAPANESE mispronounce their R's and L's in English. Why? It has something to do with the manner in which their brains process and hear the sounds. So, there are shops here on the island that have R's and L's misplaced in the names of the stores! Two examples: there's a CROTHING store up north, and there's a FROWER shop downtown. I'm serious.
The only Japanese that do not mispronounce the two are those who are fairly fluent or who may have had early experience speaking or learning English. It's very rare to find a native Japanese speaker who does not mispronounce, however. Sometimes it's fairly subtle though.