This is an interesting article that takes another look at Moses and his difficulty to articulate.
Native Tongues | The Jewish WeekMoses complains that he is kvad lashon, heavy of tongue. Naturally we assume that means a problem with his articulation. But as Rabbi Aaron Alexander pointed out to me, biblical scholar Richard Elliot Friedman draws a different, and plausible, conclusion. The same expression is used in Ezekiel 3:5: For you are not sent to a people of unintelligible speech and difficult language, but to the House of Israel. The phrase for difficult language is kvad peh.
So perhaps Moses simply did not speak Hebrew. Having grown up in Pharaohs palace, he did not know the language of his own people. When Zipporah told her father in Midian that she had met an Egyptian, it could be a result of Moses knowing only the Egyptian language. (Rashbam, the medieval commentator, says on the contrary that Moses did not speak Egyptian.)