In 1788, why was Austrias army made of many different speaking soldiers, to ultimately attack itself and lose 10% of the entire army?

Because Austria's army in 1788 was not one single and unified fighting force. Rather, Austria's army (Which at the time was not called Austria but the Habsburg Empire) was composed of squads from many smaller nations that were found in the Habsburg Empire. Because there were strong nationalist tendencies among these, they spoke differently and this eventually lead to big differences among different groups and attacks between each other.