The difficulty English learners have in hearing the difference in the pronunciation between the teen numbers (fifteen) and the decade numbers (fifty) is well documented. The 'n' phoneme at the end of a word is rare and quite difficult to say in some languages and therefor is very difficult for some people used to speaking those languages to hear.
I remember discovering this several years ago working with a group of Somali students who kept saying "twenty" after they had counted by tens to ninety. Yesterday I was interviewing a third grader form Burma and he almost made the same error catching himself at the last moment. The student has a wonderful cheerful disposition and immediately said "teen is 1 and ty is 0" as if it was something he had been taught to help him differentiate between the two. I must follow up to see if this is in fact true.
The other thing I am noticing is that many of the English learners I am working with find it very difficult to skip count by 2s an 5s. Even when they have an excellent grasp of place value up to reading, writing and modeling 4-digit numbers they still have considerable difficulty skip counting.
This apparent lack of a sense of pattern in number could be quite an issue. This is something I will begin to watch for more closely.