The first words ever written in Armenian - ճանաչել զիմաստութիւն եւ զխրատ, իմանալ զբանս հանճարոյ

This sentence even has its own Wiktionary here; it's the first few words St. Mesrop Mashtots wrote after creating the Armenian alphabet. Meaning: "To know wisdom and instruction; to perceive the words of understanding" (Proverbs 1:2). This kind of has a Zamenhofian Unua Libro-type feel to it, as a person who has created something completely new in a language now sets about actually doing something with it, and in this case wildly succeeds.

Korean is another example where the first writings in its own script are very well preserved, and of course languages such as Inuit and Cree where the script was devised by missionaries are similar in this respect.