[cromka]

Interesting point, never thought of that. As an ESL speaker, somewhat advanced and with an experience of living in the US for a while, I have learned many colloquialisms that I recently had to become more aware of when dealing with someone speaking the language at a much more basic level. So this concept actually applies to people like me, too, and maybe even more so, since we're less likely to be able to instinctively evaluate how common or rare a colloquialism can be to a another foreign ear.