I'm not sure if this has changed yet, but until recently Siri regularly failed to recognize artists, albums or titles, which often happen to be english, if the device/siri was set to a different language. In my case that's german and I can't count the number of times I had to pronounce the artists I wanted Siri to play in a ridiculously german way.

English is so ubiquitous that, in my opinion, all Siri queries should be checked for both local language and english.