Maybe, but the general would likely not be anywhere near the foreign contingents in the thick of battle, unless they were a strategic reserve, in which case im sure he would have a way to communicate with them. the truth is, battle in antiquity was not big on command and control of forces arrayed, plans were layed out before battle was joined in the case of pitched encounters, and changes to those plans after fighting had started was rare, and difficult to pull off, to say the least. the commander himself wouldnt vocally give commands anyway since theres no way his voice would carry so far to reach the varied units of a battle-line, riders, runners, sub-commanders, and trumpets and banners would be used in the most advanced, organized armies to signal the basics--retreat, advance, withdraw, charrge, and so on.