@SuppeBargeld That reminds me of an old SF book. This guy had met these very small humanoids with sort of stone age tech, and wanted to prove they were sapient to stop a corporate takeover of the planet. But they didn't seem to have language, just made sort of squeaky sounds that didn't add up to much. Then he got the idea of trying a machine that could "hear" different frequencies, and found not only did they have language, but they had learned English from him and when they talked to him, they were using it--just, mostly too high pitched for the human ear to hear.