They are nouns*
*in my layman's opinion.
While these are all correct, the same does not apply to some other nationalities:
The South Africans
I've looked up all five nationalities in the OED, and I noticed that for all five nationalities, the words are also explicitly defined nouns.
It seems to me that "the Japanese" is a noun. It stands to reason that its usage is based on the original adjective.
It just happens to be the same as the adjective, but there is a separate definition for the noun and the adjective in the OED, thus making them separate words, grammatically speaking.
Interesting to note
The first three examples all end in an "s"-like sound, which makes it hard to hear the difference between singular and plural. I have a feeling this is related, because this distinction seems to apply to every nationality I can think of.
and many, many more...
edit Here is a list of nationalities.
I've browsed the majority of the list and the presence of a plural "s" (when referring to the people) seems to always correlate to whether the nationality ends in an "s"-like sound.
There is an exception...
Thanks to GEdgar for pointing it out.
Finnish is not defined as a noun when referring to the people (only when referring to the language).
In this case, "the Finnish" would be a nominal adjective (as per Edwin Ashworth's comment), not a noun.
(Unless the absence of this defined noun is a mistake in the OED, but I can't know that based on its absence.)
- If it is defined as a noun in the dictionary, then you should consider it a noun.
- If no noun is found, but the word is defined as an adjective, then you should consider it a nominal adjective.
- If it is not defined as a noun or an adjective, then it is not correct English (unless loan words are considered correct even if not defined? I'm not quite sure about that)