I've been thinking about how a people who speak a language without rhotics would perceive a rhotic sound. Obviously of course, this would depend on exactly which rhotic we're talking about. I thought about making this question about rhotics specifically, but I think it would probably be more beneficial to talk about how languages deal with foreign phonemes in general.
From my experience, they typically just replace it with a native phoneme that overlaps with the foreign phoneme in how its pronounced. For instance, words with the voiceless velar fricative often have that phoneme replaced with the voiceless velar plosive in English. This can be seen in how the word 'loch' as in 'loch ness' is often pronounced, at least in the US.
I've also seen languages that replace an 'f' sound with a 'p' sound. Humans seem to perceive alien fricatives as sounding as plosives. I have noticed that most Americans can't hear the difference between x and k, or a voiceless palatal fricative versus a voiceless post-alveolar palatal fricative (I personally can't understand how someone could fail to hear the difference between those two, but its surprisingly common).
But some rhotics belong to unusual parts of speech. There's the alveolar flap for instance, and the alveolar trill. How do languages deal with phonemes that are part of a manner of articulation their language simply doesn't have? Personally, I suspect the most common replacements for an alveolar flap would be an alveolar approximant or plosive. They might perceive it as being somewhere between say 'l' and 'd'.
And of course there's the extremely rare English 'r' sound. European languages just replace it with their own rhotic, obviously. But what about non indo-european languages that lack a rhotic? The only non IE language I know is Japanese, which does have a rhotic (a lateral alveolar flap, ironnically). The only 'manner of articulation' I've seen any language deal with are nasals, which people seem to like to replace with voiced stops (which makes pretty obvious sense).