I would like to know if there is a rule for what is plural and what does not change to the plural for nationalities.
For example, "the French" is the same in singluar and plural,
but, Russian becomes the Russians,
and "chinese" is the same in singular and plural
usually names of people ending in -an add an s to form the plural.
Italian becomes Italians
Argentinian becomes Argentianians
Mexican becomes Mexicans
Note that plural names of people not ending with s must always be preceded by the, while those ending with s can be preceded by the or not.
French => The French
Italian => The Italian or Italian
For better explanations click here and see point 2c English Grammar
P.s: I'm just a student. If you want to be sure, wait for a teacher.