Almost certainly wouldn’t discuss them in any detail since that would require noting, as others have said, that most were communists, that the IB was organized by the Comintern, that they faced a lot of obstacles put in place by the US government on the way to Spain, etc.And yeah, would they specifically name the IBs or would my suggestion of a general reference to "brave men who went to Spain to fight for democracy" work?
Your suggestion is probably on point.
Probably something like:
Despite the indifferent policies of the democracies, not everybody was so complacent. Just as our republic would not have survived without men from foreign lands like Tadeusz Kosciuszko* who came to fight for American liberty, so thousands of men from every corner of the world arrived in Spain to give their sweat, blood, and even their lives in the cause of the Spanish people. Even a contingent from our own country came, fighting under the banner of Abraham Lincoln and swearing by his oath that government of the people, by the people, and for the people, should not perish from the earth.
*they might mention Von Steuben but considering who’s the enemy right now it might be impolitic