Thanks for the link, didn't know this was a "concept" per-se, so to speak, even less so that it had its own wikipedia page and that Wittgenstein had written about it. I've started thinking about this concept ever since I read Wilhelm von Humboldt's "The Heterogeneity of Language and its Influence on the Intellectual Development of Mankind", where at some point he mentions that, taken at the limit, each of us speak our own, very private language (the book was written in the early 1800s).

That idea has stuck with me ever since and I've tried ways of both disproving and affirming it, now I'm mostly in the "affirming it" phase. As a quick example, one can just think of a sentence like "I love you", which means different things for different people, even if spoken in the same language, and, most important of all, it might mean (and in many cases it does mean) different things to different people who have lived together for decades (as husband and wife, for example, or as parents and children). On the other hand, more "neutral" sentences like "turn off the lights" seem to mean the same thing for a larger proportion of speakers.