Hi, Harrison 18:27 Nov 7, 2011
To me, "Is English your primary language?" is one question and "¿Usted prefiere hablar en inglés?" is another. The latter may be a better question -- a less intrusive question, let's say -- to ask, but it is not really a translation of the former, which is asking about their competence and/or habits rather than their preferences. So I would say that if what you really want to ask them is which language they would prefer to use to talk to you, go ahead and do so. However, I could imagine that they might answer "English" even if Spanish is their primary language, because they could feel it's better to speak English to you, for whatever reason (perhaps because it's more polite). | 
peer agreement (net): +9