I believe that in order to make a successful translation here, you should rework the pun entirely. The wordplay in English here (are - arrghhhhh) cannot be reproduced to the same effect in French, so it is probably wiser to come up with a pun of your own that works.

How about "Tu sais pourquoi les pirates crient sur les matelots ?
Parce qu'ils hissent hooooooo!" (hissez ho ! is a notorious word used by pirates, at least in cartoons and other medias)

Or perhaps "Tu sais pourquoi les pirates n'aiment pas la Bretagne ?
À cause du Tonnerre de Brest !" (this is a Capitaine Haddock reference, perfect for Europe/France in my opinion).

Or even "Tu sais pourquoi les pirates n'aimaient pas les grandes villes ?
Car ils ne faisaient pas de quartier !"

Anyway, you get the idea. I think it is just a matter of being inventive here and reworking the pun.

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Note added at 5 hrs (2013-08-28 13:38:56 GMT)
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"Parce que tonnerre de Brest !!" works better for my second suggestion, I think. Although both are fine, but with the direct use of Haddock's "idiom" it sounds better.