ATC Watcher,
How on earth could you say that the dual language was not a contributing factor in the Shed accident?
When you are cleared to line up, you expect to have the runway to yourself. Not only that, if the take off clearance had been in English, I can pretty much guarantee that the Shorts Capt. would have stopped short and queried it. He is a very careful chap and only survived the accident as he was leaning forward to try and look up the runway when they were hit. Unfortunately the FO wasn't so lucky.....
We can never say for certain if a single language would have prevented the accident, but I'll bet money having one language cuts the chances of something similar happening again. A change I think should be made as soon as possible.