I had a serious scare at an airport with parallel runways when ATC sent a French-speaking aircraft across in front of me (like a few hundred metres in front) to join the LH runway while I was on final for the RH. He'd spoken to me in English the whole time, to the other chap in French the whole time.
The ATCO may have had situational awareness, but I don't think the Frenchman or I did.