The Fremen are not Arabs. The Arab people are among their several very, very distant ancestors, and their culture is a hodgepodge of multiple cultures and religious movements. We're talking about a mixed culture that has survived, separately from Earth, for thousands of years. For their words to be pronounced like they are today is absolute nonsense. Heck, we're less than 2000 years separated from the Roman Empire, and we have no way of truly knowing how Latin was spoken back then, despite the language still being taught and used. Your expectations are unreasonable if you want the Fremen to be a perfect replica of modern Arab culture and language or even physical appearance. Come on, don't make grand sweeping statements based on the trailer alone. "Jihad" isn't replaced with "crusade" in this movie. Both words are used in the book, after all. The movie doesn't use the word "jihad," however, at least from what I can remember. The word is instead replaced with "holy war."