I think it really depends on the client. If they are evaluating sales to local residents, it may be required to speak the local dialect. You just have to ask.
I have taken some restaurant assignments where it was requested for me to order in French, which I am comfortable with. The problem is that the staff can clearly tell that I am American and most will quickly shift to English after a few sentences when they hear my accent, so I cannot really perform the shops in the local language.
When I questioned the MSC, the told me it was acceptable, provided that I was able to speak French if needed; In case there were some staff that could not speak English (and some of the bussers could not). The reason for the rule was to prohibit shoppers that could not speak French at all from taking the shop and then commenting that they had poor service.