Freeciv

Did some tests in Civ3 Conquests and found a discussion: https://forums.civfanatics.com/threads/how-do-you-select-the-population-unit-to-pull-for-a-worker.447568 Actually, in Civ3 no button found to select from whom to build settler. Two pops (one for worker), likely leftmost from the pop-faces row and a random (?) another, are taken, second one determines the nationality of the unit, the nation of the other one is forgotten; if you have only one your civ citizen, the settler will be foreign. That does not mean we need it exactly the same.

For freeciv the main effect is probably that with some rulesets, when you build a settler from a conquered city and then found a city with that settler, the newly founded city is going to be unhappy with you (the enemy).

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- "diff = MAX;" - I guess you meant to use MIN instead of MAX here.
- "city_nations[selected].change = (change += diff);" > divide two two lines for clarity (first assign 'change'). For a moment I thought there was a bug here.
"struct player_slot *city_nations[MAX_CITY_NATIONALITIES];" on citizens_update() would require other changes as well, unlike the one in citizens_convert(). The difference between citizens_convert() and citizens_update() cases is that former puts in only those nationalities that actually exist in the city (and thus cannot exceed MAX_CITY_SIZE) "if (nationality != 0) {" and latter all the nations.