Solution found in another forum :

Don't connect the LCD to the SPI pins. Pins 10, 11, 12, and 13 are NOT available for you to use.

By using the pins 4 and 7 instead of 11 and 12. All works well

As you say, I can't use pins 10, 11, 12, 13 because they are used for the SPI.

-> So, I tried to use the pins 4 and 7 instead of 11 and 12.

The problem is that the pin 7 is used by the Wifi shield (Handshake) and I have no other free digital pin.

-> So, I tried to use an analog pin instead of the pin 7 and ..... it works very well.

Here is my final solution :

• Arduino Pin 2 -> LCD Pin 4 (RS)
• Arduino Pin 3 -> Keyboard IRQ
• Arduino Pin 4 -> LCD Pin 11 (D4)
• Arduino Pin 5 -> LCD Pin 12 (D5)
• Arduino Pin 6 -> LCD Pin 13 (D6)
• Arduino Pin 7 ->
• Arduino Pin 8 -> LCD Pin 14 (D7)
• Arduino Pin 9 -> Keyboard Data
• Arduino Pin 10 ->
• Arduino Pin 11 ->
• Arduino Pin 12 ->
• Arduino Pin 13 ->
• Arduino Pin 14 (= Analog 1) -> LCD Pin 6 (E)