Language: Erlang

Source code:

-module(task). -import(string,[right/3, left/3, sub_string/3, concat/2]). -export([ping/0, pong/0]). ping() -> Pong = spawn(task, pong, []), Pong ! {self(), ping}, receive pong -> pong end. pong() -> receive {Ping, ping} -> Str1 = "This is a flag", Str2 = right(Str1,4,$.), Str3 = sub_string(Str1,5,7), Galf = concat(Str2, Str3), io:fwrite("~p ",[Galf]), Ping ! pong, Ping ! Galf end.

Run it, or try to read the code

The flag is what ping() function returns