Borrowing words from Japanese is oddly tricky because Japanese is zero plural, meaning they don't modify the word from singular to plural. You can see in Japanese heavy English scripts like Last Samurai, when they borrow english words, they don't modify them. ie "we are samurai," "the ninja are coming." en.wiktionary.org/wiki/kaiju
But all this is just linguaphile stuff, I wouldn't be surprised if the film adds an s to pluralize kaiju.