Lets say some slaves taken as prisoners of war were captured from Gaul or Germany and brought to the Italian Peninsula to be sold to Romans. There is obviously a language barrier. Would the Roman masters teach or have someone teach their slaves Latin, or at least the basics? Or would it just be up to the slave to learn the language or be punished for not understanding the commands given to them?
If it's anything like the situation in Mediterranean slavery in late medieval to early modern times, larger households had quite a few slaves, so a new slave in assigned the bottom tier job unless he or she has special talents. In either case, they have to learn how to receive instruction, which was often given in a polyglot language called lingua franca, referring to Levantine and North African overlords referring to all Christians as "franks". This is a mix of French, Italian, Spanish, etc.
Those who could learn their master's language get the opportunity to get better less demanding work.
If a slave ends up a galley slave, then the instructions are minimal, and the slaves speak among themselves using lingua franca.