A 12th-century abbess invented her own language
Hildegard of Bingen — abbess, composer and natural-science writer — also devised the Lingua Ignota: a vocabulary of roughly 1,000 invented words, accompanied by a companion 23-letter custom alphabet called the litterae ignotae. It is often cited as one of the earliest known constructed languages.
How we know
Hildegard (1098–1179) referenced her 'unknown language' in a letter c.1153–54 to Pope Anastasius IV; the surviving Lingua Ignota glossary contains ~1,011 words (mostly nouns, with a few adjectives) recorded in Latin characters in the Wiesbaden Codex and Berlin MS, paired separately with the litterae ignotae (23 custom symbols); described as one of the earliest known constructed languages (Wikipedia; World History Encyclopedia; Univ. of Notre Dame manuscript blog).
One of many fact-checked wonders across eight traditions at The Spring of Wisdom.