False friends
I heard an interesting linguistic issue on a Langen sector on Wednesday morning: the controller said "When you can be level xxx by yyy, descend level xxx". Since the aircraft in question was heading towards yyy at great speed, "when" didn't make much sense in that context, and the crew were consequently confused by the instruction, which was repeated unchanged.
This must be a difficult one for native German speakers: "if" in English is "wenn" in German. Any other examples of linguistic "false friends" in an ATC context?