I am an ardent proponent of promotion of Urdu language. However, I think we should speak English if the official language at workplace is English. Where there are no rules in regards to the office language, one may use the language he/she is more comfortable with, which obviously is the mother tongue.

I never speak English with any Pakistani who I know can communicate in Urdu. Nonetheless, I never speak in Urdu with a friend or a colleague when a third person is present whose language is not Urdu.