I run Wireshark on a MacBook Air and I also have a Android phone on the same WiFi-network. If I visit a website with the Android phone I can't see that DNS-traffic (or other traffic) in Wireshark on my MacBook Air. Is there any way I can capture all traffic on the WiFi-network in Wireshark?
You need a network card that can be set in 'promiscuous mode' which I don't believe the MBA allows.
You may have better luck using ADB to get logs via usb debugging in this specific case but in general you're going to need different network hardware.
You'd need to capture in promiscuous or monitor mode; not all Wi-Fi adapters and their drivers handle promiscuous mode, but many of them handle monitor mode, including the adapters in various flavors of MacBook and the OS X drivers for them.
If the network is encrypted (WEP/WPA), monitor mode traffic will be encrypted, and you will have to arrange that Wireshark can decrypt it.