Politeness, surely. Almost always, in every language, degrees of politeness are expressed by greater formal complexity and less clarity. This is because, as diplomats know, clarity and straightforwardness, bluntness, make disagreements and the disagreeable more obvious.

Polite or diplomatic speech violates the rules of plain language, taken completely literally.

But besides the likelihood of disagreement between cultures, politeness, or more broadly what we call the phatic or emotional element of communication, is also far more important in most other cultures than it is in English. Anglo-Saxons are, to be blunt, unusually blunt.

Here is a lovely example, a poster from the Butuan airport, in the Philippines. It is purely phatic.

What is a Filipino Policeman?

A Filipino policeman is a protector and friend of the people. His badge is a symbol of the citizens' faith and trust, his uniform a mirror of decorum and integrity and his whole human person an oblation of enduring love for homeland, fellowmen and God.

A Filipino policeman emulates the valor of Lapu-Lapu, the serenity of Rizal, the leadership of Aguenaldo, the courage of Bonifacio, the idealism of del Pilar, the wisdom of Mabini and the fortitude of Gomez, Burgos, and Zamora.

A Filipino policeman has an oath for his republic to uphold: to defend the constitution, honour the flag, obey the laws and duly constituted authorities. He has a covenant with his people to comply: to safeguard and protect them even beyond the call of duty. And he has a legacy for his family to fulfill: to bequeath unto them the one and only treasure of his life--an unblemished name.

How would you edit this for plain language? If you boiled it down for information, you would have almost nothing left: "A Filipino policeman should be a protector and friend of the people." Full stop.

This would clearly be wrong, and meaningless, in Filipino culture.

I would tinker only with the last paragraph, and for the sake of grammatical correctness as much as plain language: "A Filipino policeman swears an oath to his republic," and, in the second sentence, "He has a covenant with his people: to safeguard and protect..."; "And he leaves a legacy to his family: the one and only treasure of his life..."

Everything else might be debatable, but not on grounds of plain language. It all serves a phatic purpose, and the phatic use of language is essential in other cultures.