I’ll quote what someone else in that position said (originally posted about on my blog: .sig quoting Marcel Cohen, corrected; see also Language Regained).
Marcel Cohen was a Jewish author writing in French. His first language was Judaeo-Spanish (aka Djudio, Ladino), which he barely remembered as an adult. As a one-off, he wrote a memoir in Ladino in 1985, with a parallel French translation.
At the start of the book, he writes what it feels like to use a nearly extinct language:
“Dear Antonio. I’d like to write to you in Djudio, before the language of my ancestors is completely extinguished. You can’t imagine, Antonio, what the death agony of a language is like. You seem to discover yourself alone, in silence [every day that God grants you]. You’re sikelioso [sad], without knowing why. What I’m going to record here is more or less what my mind retains of the five centuries that my ancestors spent in Turkey. I was born in Asnieres, a suburb of Paris, and my parents were in their thirties when they came to live in France. They spoke French perfectly. At the time it was the language of all the Jews of the former Ottoman Empire. They learned it at an early age in the schools of the Alliance israelite universelle, then in Istanbul at the Lycée de Galata Sarail. How could they not have loved France. This didn’t by any means stop them from speaking Judeo at home. And so it was that listening to them I was immersed in the language, without exactly speaking it myself.”
The phrase in brackets was left out in the French translation by the author: it was something the author felt that a Ladino-speaker could say, but a French-speaker could not (Laïcité and all that.)