First time here; I discovered this thread via Google whilst trying to find the answer of the film's renaming myself. Having not found anything conclusive, I did some more digging and believe I've identified the answer.
If you look the film up on IMDb and go into the page listing international release dates and titles, one needs only a basic understanding of language to deduce that Salazar's Revenge is the title everywhere except North America and Portugal.
In other words, DMTNT is the exception, and SR the rule. It doesn't take a genius to deduce, then, that Salazar's Revenge must be Disney's preferred title for the film, but something stopped them calling it that in those two territories noted above.
To put it another way: the original question here, and on other similar posts around the web, is looking at the name the wrong way around. Salazar's Revenge is the main title, with Dead Men Tell No Tales being the renamed, secondary choice.
So why would Disney not use SR in those territories? A quick Google provides pretty compelling possibilities.
In the case of Portugal, the country's 100th Prime Minister, in office for the incredibly long period of 1932 - 1968, was named Salazar.
For North America, a man named Ken Salazar was Secretary of the Interior under Barack Obama from 2009 to 2013, and was to play a key role should Hillary Clinton have won the Presidency in 2016.
There are references to Dead Men Tell No Tales having bean POTC 5's working title since at least 2013, if not earlier, so it doesn't seem an unfair assumption that, feeling they were unable to use the preferred subtitle for the film, they simply stuck with what it was already being referred to as.
I have no association with Disney and can't say in complete certainty that my theory is correct, but it seems compelling enough to me to be worth sharing. I hope someone finds it helpful, or at least of interest.