There are other lines of evidence. I don't know much about documented cases of feral children, but presumably there must have been at least one known case that developed to some meaningful age at which thought was obviously happening in spite of not having language. There are children with extreme developmental disorders delaying language acquisition that nonetheless still seem to have thoughts and be reasonably intelligent on the grand scale of all animals if not all humans. There is Helen Keller, who as far as I'm aware describes some phase change in her inner experience after acquiring language, but she still had inner experience before acquiring language. There's the unknown question of human evolutionary history, but at some point, a humanoid primate between Lucy and the two of us had no language but still had reasonably high-order thinking and cognitive capabilities that put it intellectually well above other primates. Somebody had to speak the first sentence, after all, and that was probably necessary for civilization to ever happen, but humans were likely quite intelligent with rich inner lives well before they had language.