What was the ethnic makeup of the Roman army towards the end of the Republic and through the Empire period?

As the number of citizens willing to join the legions was smaller than the needs, all Roman rulers - starting with Julius Caesar, who enrolled Germans and Gaulls in his army - began to use more and more barbarians in the army, offering them the virtual citizenship, namely to get it after finishing their military engagement. Only the praetorians were Italic. And as the legions were deployed on the borders, there was their recruiting areas, too -Gaull, Hispania, Syria, Brittania, Thracia and so on. Of course, the quality of those new soldiers was not at the high Roman standards, there were no Romans to fight for their country, but mercenaires: and things got even worse after entire barbarian tribes were enrolled in the army.