Although maybe the japanese would be first to deploy 6th-gen if they can reach their stated 2031 date.
First?
The problem is what we understand by generation, as always.
Apart from advertisement clickbait, there are 3 logical approaches here:
1, technology-based: 2031 aircraft won't really belong to the same generation as a, say, 2006 F-22, unless there is something
fundamentally wrong with the capabilities of the country designing it.
There are such approximate "sets of technological solutions", characteristic for generations of aircraft - sometimes they're used(3rd gen), sometimes they for some reasons are just skipped(no one gave to 1990-2000s fighters a generation - when they actually deserve one at least as much as the 3rd gen did).
2, capability-based: aircraft must be
so capable, that it literally breaks the world into "before" and "after", i.e. dreadnought moment of sorts(like F-15 did in the mid-1970s). Will Japanese fighter be
so advanced to clearly devalue 2020s fighters of other nations?
Maybe? As of now, I am
very unsure of it.
3, country-based. Will Japanese fighter represent a clear step up from both F-2 and F-15JSI? Almost 100%.
So, for local air power calculations, it will clearly be a new, 2030s generation. 5th, 6th, or 26th one? Doesn't matter really.