Old Phoenix in Poul Anderson's "Losers' Night"?
The first who speaks:
must be a loser - or he would not be here on this occasion;
is lean, dark and somber;
speaks classical Latin;
wears a toga;
expects to fight a battle soon, probably at Philippi;
aims, after the battle, to restore the Republic.
That Republic will not be restored. Instead, an Empire will be built. This must be Brutus?
answers the same physical description;
speaks Canadian French;
wears a plaid shirt and faded blue work trousers;
realizes who the other man must be;
claims to be a simple, unlearned metis;
is advised to return to his country because his people need him;
replies that they cry to him for justice...
Is this Louis Riel?
Clearly, I have not read this story closely enough before.