According to a Politico report, Donald Trump has trouble pronouncing the names of (some) foreign countries and also had to have time zones explained to him when trying to reach foreign leaders. 

That report states that when Trump was studying a briefer's map of South Asia prior to a 2017 meeting with India's Prime Minister, he thought that Nepal was pronounced "nipple" and pronounced Bhutan as "button."

"He didn’t know what those were. He thought it was all part of India," said someone familiar with the exchange. "He was like, ‘What is this stuff in between and these other countries?’"

A Trump national security official official also stated that he avoids certain words/names when speaking to a foreign leader because he's not sure he can pronounce them, while a White House official added that he tries to be respectful and get pronunciations right. 

As for the time zone thing, an ex-Trump official said "He wasn’t great with recognizing that the leader of a country might be 80 or 85 years old and isn’t going to be awake or in the right place at 10:30 or 11 p.m. their time." Another person reportedly close to the president added to that by saying "When he wants to call someone, he wants to call someone. He’s more impulsive that way. He doesn’t think about what time it is or who it is."

In his defense a White House official says that Trump is aware of time zones (which is good) but doesn't worry about them when he wants to contact a world leader. "He’s the president of the United States. He’s not stopping to add up," said that official. "I don’t think anybody would expect him or Obama or Bush or Clinton or anybody to do that. That’s the whole reason you have a staff to say ‘Yes, we’ll set it up,’ and then they find a time that makes most sense."

You can read all about this diplomatic learning curve at Politico, if you want.