How can one reconcile the Hadith of enjoining good and forbidding evil with the Hadith of leaving what does not concern a person? And if a Muslim sees an evil in an Islamic country, such as girls meeting (in a way that is considered unlawful), should he advise them despite knowing they will not respond, or should he report them to the authorities?

There is no contradiction between the hadith "Whoever among you sees an evil, let him change it" and the hadith "Part of the perfection of one's Islam is his leaving alone what does not concern him." Each applies to a different situation. Enjoining good and forbidding evil is an obligation, while a Muslim's lack of interest in what does not concern him falls under leaving alone what does not concern him.

The denunciation of evil should be to the extent of one's ability: with the hand if possible, without causing a greater mischief; or with the tongue; or with the heart, which is the weakest form of faith. What "does not concern" a Muslim is not among the obligations or recommended acts; rather, it is what exceeds the needs of religion and worldly life, or what does not benefit in gaining Allah's pleasure, and it includes curiosity in speech, actions, sight, and thought.

If you see an evil, you should denounce it directly to the perpetrator if that is possible to prevent its continuation, or you should inform the authorities if the evil is significant and you cannot change it yourself. The goal is to remove the evil, whether by your hand or by someone else's.