You can assure no such thing.

I live in a Gaelic speaking area, one man I know who is actually named Donal but is called Dougal by everyone.

However Donald John Trump is neither living in my gaelic speaking area or on Lewis in the 19th century. He was born and raised in America, and in America he was named Donald John. His parents choice.

If they had wanted him to be known as Sam or Fred they would have named him accordingly, they didn't, they wanted him to be known as Donald John.