In Medea, why does the Chorus praise the city of Athens?
I don't see the Chorus' praising of Athens as something in the form of nationalism or political zeal. Their praise of Athens is more of a validation of how moderation and a sense of control must exist within people of a community in order for a community to function and prosper. Their praise of Athens presents Euripides with another path to show how people wronged can pursue their own feelings of anger and resentment. Athens is shown to be a source of wisdom, judgment, and grace. The Chorus praises Athens precisely because Medea's plans to murder her children represent the exact opposite.
The Chorus operates in this moderate vein. They are the force that supports Medea as a woman, but distances themselves from what she does. The Chorus is very quick to criticize Jason and support Medea for feeling wronged. Yet, when the Chorus speaks out against Medea's vengeance that embraces the killing of her children, the Chorus cannot support such action. When the Chorus hears of her children being killed, the Chorus speaks out against what is happening:
She fell, poor woman, into the sea, in an
impious murder of her children,
stepping too far on the promontory,
and she perished, dying with her two sons.
After this, is anything too horrible to happen?
O bed of women, site of many labors,
how many evils you have already
brought to humanity.
The Chorus operates as a force that speaks to vengeance as being something that cannot be tolerated in a civil society. The Chorus of Women praises Athens because the vision of social progress and elevated thought that it represents is superior to the base and savage instincts that Medea embodies. In their praise of Athens, the Chorus, while supporting Medea's anger, seeks to raise her condition from one seen as an animal to something more of a human. The Chorus praises Athens, invoking it as a potential for what can be, as opposed to Medea who embodies more of what sadly is for human beings.