It can be confusing, caution is in order.
As a permanent resident of Mexico for almost 10 years and who drives Sonora Mexico plated vehicles, we have been taught that on the highways if there is someone behind you that wants to pass and it is clear for the to do so, then you signal that to them with your left turn signal.
When you are in the city, that rule usually is not followed as was mentioned earlier there are lateral roads in some cases and often left turn lanes.
We have driven more than 50,000 miles in our time in Mexico from the Baja to most of the mainland and we have never had an accident or even a close call.
When we traveled to Spain a few years ago we almost got mucked out because I applied logic from Mexico to Spain. I learned very quickly how the rules work in Spain and I followed them to the best of my ability even though it was counter-intuitive to me. So I guess the point is to figure out the country rules and try to follow them without complaining that it is wrong and they should do it our way....