A German friend of mine once told me what Midwestern English speakers sound like to her. It was basically: "wee ta wi ri a we". I told her that German sounds something like "bloff grush liben" to us. My wife is Chinese, and I work with a lot of Mandarin speakers. I recall when I first heard Mandarin, I couldn't tell where one word stopped and the next one began. It's weird, but my brain hears it completely differently now. Even though I only understand a small amount of Mandarin, my brain breaks up the sounds into words somewhat automatically.
This also happens with children. It seems they first learn to hear the sounds of a language before they are able to speak it. Take this example: So the child hears the difference between the s and th, even though she can't pronounce it herself.Recently a three year old child told me her name was Litha. I answered "Litha?" "No, Litha." "Oh, Lisa." "Yes, Litha."