Why is it so hard to count this way?

I've done some reading on neuro-plasticity over the last few years and been dabbling in cognitive testing for assessment and selection. What appears to be happening is the neuro pathways responsible for language are separate enough that completely different areas of brain must be called to do both things. Most people are typically better at one or the other, but not both. Because the areas of the brain that do math/sequence and verbal expression are different, there is lag time between the cognitive processes as one process must be completed before the other can be used. Also to add support to Amy's comment, I had a German teacher in high school who claimed to have difficulty sticking to one language in conversation with locals when she would visit home (Germany).