Is Speedbird a legal airline name?
Yes
So we can use any radio callsign of the real world ?
Speedbird = British Airways
Some other can be interesting
BMI Regional =Kittiwake
Virgin America = Redwood
US Airways (old)= Cactus
etc...
Normally Yes.
Becuase those are callsign common "nicknames", but are not a registered trademarks or property.