It's quite ambiguous, and fortunately so!
As shown by existing answers, director Rian Johnson seems to have a clear answer to the question, but that doesn't make the title of his movie less ambiguous, especially since many of the film's official translations deviate from his opinion (neither do we know who was ultimately responsible for the title anyway). Of course the decisions of whatever international marketing division aren't necessarily more significant than the director's interpretation either, but this unclarity even among the official channels adds to the high ambiguity of the original title. Seeing how that ambiguity is just centred around the specificities of the English language and the declination of a fictitious word, it might be entirely coincidental and not intended by the film-makers at all, but it is still very much there, and quite apparently so (as your question and the interest it generates shows afterall).
We could argue if Luke Skywalker is the only Jedi left or if there are technically other Jedi around. But this is also made quite difficult by the unclarity what a "Jedi" actually is (which isn't helped by the institutionalized arm of the Jedi having been destroyed with the rise of the Empire at the end of the prequel trilogy, and the sequel trilogy bringing much more of the fuzzy mysticism back into the Jedi idea). Is it any force sensitive person? Is it some force sensitive gal carrying a bunch old Jedi codices around with her? Or is it only someone who went through a proper 4 year B.Sc. programme in Jedi?
But the ambiguity doesn't just end there. As other answers have shown, the films themselves do indeed use these words (in the title sequences as well as in dialogue) to describe a specific individual with them, Luke Skywalker that is. But then again, the mere fact that Luke might indeed be the last of the Jedi makes him also pretty much a representative of the very Jedi religion itself. So he might very well incorporate singular and plural in unison.
But beyond arguing about what a Jedi is, how many there are left, or what whoever from Disney thinks the title refers to, there is a strong theme of ending/renewing the era of the Jedi that drives itself through the movie. Afterall, Luke himself (at least initially) wants the Jedi to end for having grown arrogant and Kylo Ren actually agrees with him in that (as does Yoda to some degree). Although, the film isn't entirely uncompromising in its "out with the old one, in with the new one" attitude, especially when it has Rey save the old Jedi codices, there is a strong sense of, if not ending, then at least renewing or revising the Jedi idea. And you might argue that the notion of the entire Jedi era ending is represented more by the plural than by the singular, but that alone could very well just be a subjective impression, too.
What all these ramblings are hopefully showing is that this isn't really a simple question to answer. But it's also not a simple question to ask either, seeing how the ambiguity adds to the theme of ending the Jedi as well as, in its referral to Luke himself, to the question what Luke actually means for the Jedi as a whole and ultimately to the question what "Jedi" and being a Jedi really means, nothing of which is simple to answer. So even if that looks like a lame excuse, we might just resort to the platitude that the question itself is the answer. Coincidental or not, it's a really interesting question and Rian Johnson might either have been joking or a little bit too dismissive in his judgment of it. ;-)