I think it's the language that allows a person to communicate with the people they want to interact with (including anciet people who wrote anciet/classic texts) :)

Latin

Whatever it is, I think you know it or are learning it. haha :) Honestly, I would say Arabic, but I am biased, because I am Muslim and it is the language God spoke to us.

Depends where you are. I would say English as it is a universal language and can be used to communicate in almost every country in the world.

I agree that it depends on a situation. But I would say that speaking English has always been really helpful for me as far as it is an international language.