The L sound has been a long standing sticking point for English learners from Japan because it’s formed almost the precise way Japanese R sounds, which involve the tip of the tongue flicking the roof of the mouth.
The main difference, for those not privy to the mechanics, is this: the L sound places the tongue’s tip for the beginning and middle parts of forming the sound, where the Japanese R sound only places it there for the middle portion (assuming it’s the crisp type of R sound, and not the soft, growl-like R familiar to English speakers). It takes lots of time, lots of practice and lots of getting wrong to ingrain these into the tongue’s muscle memory; as anyone ridiculed for not sounding like a native speaker will tell you, it can shred on your nerves when you have to learn it all from scratch.
Consequently, since L sounds aren’t part of their phonetics, R sounds are used to render words like Light(ライト) and Lonely(ロンリ), and since folks in Japan don't care much, their L sounds will be somewhere between what we know as R and L sounds, some people on one extreme, and others, on the other