Where does “Latn” come from?

Latn (Translingual) comes from English Latin, from Middle English Latyn, from Old French latin, from Latin Latīnus, from Latin -īnus, from Proto-Italic -īnos, from Proto-Indo-European -iHnos, from Proto-Indo-European -no- — adjectival suffix.

Ancestry of “Latn”, step by step

Latn traces back through two separate lines of ancestry.

via English Latin

StepLanguageWordMeaning
1EnglishLatinOf or relating to the language spoken in ancient...
2Middle EnglishLatynRelating to the Latin language; Relating to the...
3Old FrenchlatinLatin language
4LatinLatīnusLatin; of or pertaining to Latium, the Latin language, people, or culture
5Latin-īnusof or pertaining to; -ine
6Proto-Italic-īnosof/belonging to
7Proto-Indo-European-iHnosCreates adjectives of materials
8Proto-Indo-European-no-adjectival suffix

via French Latin

StepLanguageWordMeaning
1FrenchLatinLatin; a person from a Romance country such as...
2Middle FrenchlatinLatin language; Latin
Every word from Proto-Indo-European -no-Every word from Latin -īnusEvery word from Latin Latīnus