Camp (French)
Ratisbon = Regensburg
Battle of Ratisbon, April 23, 1809—Napoleon defeated the Austrians. Browning uses the French name; the city is now Regensburg, Germany.
Marshal Lannes
Jean Lannes, one of Napoleon's most trusted marshals. He was actually wounded at this battle and died eight days later—the historical irony Browning's playing with.
flag-bird
The French eagle standard. Napoleon replaced royal flags with eagles modeled on Roman legions—the boy is saying he planted it in the town square.
"I'm killed, Sire!"
The grammatical precision matters: not "I'm dying" (future) or "I'm wounded" (survivable), but "I'm killed" (already done). He corrects Napoleon's euphemism with military accuracy.