It is a lack of distinction issue. It is not the surface issue, but it is a major cause for it. For people whose native languages don't make the distinction -- and especially if the two sounds are similar -- in their own language if you pronounce it either way, it won't make a difference in meaning. In their minds, they are likely to perceive the two sounds as merely variants (allophones) of one identical sound (phoneme). So whether they pronounce it one way, or another way, it's all the same to them psychologically -- until they are specifically trained or made consciously aware of it (such as if they are taking a course in Dutch and are serious about improving their Dutch language skills). When they learn a new language (especially if they are beyond a certain age), they usually impose the system of their native language onto the new language.

I'm sure you've noticed plenty of people having trouble acquiring a good accent in a different language (especially if they learned it later in adulthood). Their brains, their muscles, their ears, are all hard-wired to only respond to and produce the sounds of their native language(s). Most people upon learning a new language will perceive, assess, and produce the new language through the lens of the system of their native language; some people even do that for the rest of their lives.

That's why you can have a group of speakers failing to distinguish two sounds in another language. It is like some French people who can't pronounce the English TH, and use S or Z instead, because they don't have TH sounds in the French language. They will likely mix up English TH and S/Z all the time (and pronounce both as S/Z or something S/Z-like).

Did any of that make sense to you?

• "There is another reply here saying excactly the opposite." --> Most responses agree with my observation. At least with regards to (standard) Mandarin speakers who go to English-speaking countries. I don't know if they get other influences if they go to a country where they speak a different language in everyday life, where they may mix up 3 languages together.

• "I am not talking about all chinese people. I AM saying all people mixing them up are Chinise (allthough I cannot be 100% sure of this, but for the sake of clearity)" --> When you say "Chinese speakers" without clarifications, people are going to assume you mean all Chinese speakers. And as many other posters have pointed out, Koreans and Japanese people also mix up Rs and Ls frequently. So clearly, not ALL people who mix up Rs and Ls are Chinese. The point of talking about Chinese people of different linguistic backgrounds was to point out that there are many variables involved. Just because you personally found a lot of Chinese people to mix Rs and Ls and find a lot of people agreeing with your observation impressionistically doesn't make that necessarily a truly widespread phenomenon applicable to all or only Chinese people.

• "I fail to see the relevance of you pointing out that Rs are pronounced differently in dialects within Dutch or German." --> That was just a disclaimer that not all Germans and Dutch people pronounce Rs necessarily the same way. A Chinese person who is in constant contact with, say, Northern Germans who pronounce R one way may "mix up" the R in a different way than another Chinese person of the same language background who is in contact with, say, Austrians who pronounce R another way.

While it's true most of us here talk from anecdotal experience, some of those who are more trained in linguistics or Chinese linguistics have been trying (unsuccessfully, apparently) to explain how the phenomenon you observe might have come about (with some disagreements with you about details and assumptions of your observation, and attempts to clarify them).