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With all my respect to those who have answered previously, I would dig deeper than that, language "any language!" is a part of the communication process, that's why I would say the most friendly language that everyone understand is the Nonverbal Communication.
We "Human being" use gesticulation by arms, body movements, face mimics, body postures, space around us, our clothes and many other elements of communication. This is how you can deliver an information if you don't know the language itself.
But If you insist, I would simply say "English", even if your English dictionary isn't that good, you can establish a conversation or communicate using those words that you know "in English" and reach some sort of request and reply "connect to others".
Based on our history "Global history", the advancing technology is forcing almost everyone to learn or at least speak some English. So yes English now is the friendliest language that you can use everywhere and anytime.
it is different for individuals. for me Russian and Arabic are rich languages. English cannot be labelled as a language- it's mixture of languages. for majority the choice would be English but i feel comfortable with Arabic and Russian. None of which is my mother tongue.
