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How much will it cost to change my keyboard from French to English?

On that model keyboard is integrated in the upper case. As it’s considered vintage you won’t find a new replacement, thus you’ll have to revert to second hand market where you’ll find different spares conditions and prices. Replacement isn’t exactly easy, you’ll have to remove quite a bit of screws, but it can be done with some patience..here’s a guide to help you with the task: MacBook Core 2 Duo Upper Case Replacement