What is the difference between an "athar" (narrative/report), a narrated "khabar" (account/report), and an enduring "sunnah" (prophetic tradition/practice), and what is the meaning of "athar" in the saying "Whoever learns the language of a people is safe from their evil"?

Linguistically, al-khabar (report/news) refers to that which is narrated and can be either true or false. In the terminology of the Muhaddithun (scholars of Hadith), it is sometimes considered synonymous with al-hadith, and sometimes it refers to what came from other than the Prophet (peace be upon him).

Linguistically, al-athar (trace/narration) refers to the remainder of something. In the terminology of the Muhaddithun, it is applied to what was narrated from the Prophet (peace be upon him) or from his Companions regarding the Sunnah, or it is applied exclusively to what was narrated from the Companions.

Linguistically, al-Sunnah (tradition/way) refers to the path that is trodden or followed. In the terminology of the Muhaddithun, it refers to everything attributed to the Prophet (peace be upon him) of saying, action, tacit approval, or description.

As for the mentioned narration, "Whoever knows the language of a people is safe from their cunning," it is not a Hadith from the Prophet (peace be upon him). However, its meaning is considered sound by some, and it encourages learning foreign languages when needed, just as the Prophet (peace be upon him) commanded Zayd ibn Thabit to learn the language of the Jews.