What is it called when you refer to a noun by just its adjective?

I think the term you're looking for is Nominalized adjective, according to Wikipedia:

A nominalized adjective is an adjective that has undergone nominalization, and is thus used as a noun. For example, in the rich and the poor, the adjectives rich and poor function as nouns denoting people who are rich and poor respectively.

In your sentence a group of Chinese can be read as a group of Chinese people and Japanese can be read as Japanese food. The latter can be done for many countries with a particular cuisine, some examples (the [food] in brackets is optional):

Let's eat Italian [food] today.

Do you like Chinese [food]? Not really, I prefer Thai [food].

Shall I order Mexican [food]? Not today, I'm more in the mood for Indian [food].

It might seem weird because it can be interpreted in other ways (for example if you replace the [food] with [people] in the examples above), however, that is the case with a lot of words. A lot of words have multiple meanings, but it's mostly clear from the context the words are used in.

Attribution: "Nominalized Adjective." Wikipedia. April 04, 2018. Accessed April 05, 2018. https://en.wikipedia.org/wiki/Nominalized_adjective.