Why do instruments from different countries not sound good together?

One of the reasons is that not all instruments are harmonic. If you were to take the example of a Javanese gamelan, they use a lot of metallophones. Because they are metal bars, they don't strictly have harmonics. In fact they're what we call partials. Rather than having a frequency at which they vibrate and harmonics above that of one, two three or four times, the modes of vibration are no longer integer multiples, so 1.54, 1.76. Hence they make a wonderfully rich sound. But when we play them with other instruments that are harmonics, the harmonics and the partials don't coincide with each other and that causes a very confusing pattern of vibration inside the cochlea and the inner ear. It's this which leads to problems with tuning between these two different instruments.