During the course of the film the protagonists travel between multiple worlds and encounter numerous people in vastly different environments. How is it that everyone speaks the same modern English language?
Believe it or not, contrary to most other fantasy realms, there is an extremely concrete reason why the characters speak English.
As per the books (to which the movie is stated to be a sequel, technically) the Dark Tower series is, in itself, created by Stephen King, who is an English speaker. Since Stephen King wrote the characters as English speaking with minor changes, the characters speak English.
The reason this is actually an explanation is because Stephen King writing the books is an event in the books. Roland actually talks with King at one point and tells him to write the last few books, and King writes them all in English.