You have to set your WLAN country code before flashing if you plan to use WiFi on your Raspberry! Failing to do so may lead to your Raspberry disturbing other wireless services or even aviation radars in the area.

1. Open the graphical Raspberry Pi Configuration application from the Preferences menu.
2. Select the Localisation tab.
3. Click on the Set WLAN Country... button.
4. If your country is not already shown, click on the button next to the text Country and select your country from the drop-down menu.

This will configure your Raspberry to use only the WLAN channels allowed by the local regulations.