i dunno, i feel like you could teach anyone to make an "l" sound ... maybe not though
It's a lot harder than you think, especially if you learn a language past early adolescence. Overcoming the nuances of your main language become really difficult at that point because they're so deeply ingrained into your verbal functions.
I'm not saying it's impossible, I'm just saying that most people don't have the time or need to go that far when learning a language. I mean, when's the last time you've met a foreigner who wasn't Greek that really went out of their way to learn how to pronounce the various "th" sounds correctly?