Why do we say . . .
The Russians are here.*
The Koreans are here.
The Americans are here
The Italians are here.
The New Zealanders are here
The Pakistanis are here.
but we say . . .
The Chinese are here.*
The j__anese are here.
The British are here.
The Swiss are here.
The French are here.
The Portuguese are here.

It's mostly phonetic. That is, it depends on the final sound of the word.
Words ending in

-an, -ian, -er, -i

require an s in the plural Words ending in

-ese, -ish, -iss, -ch

do not change

Languages (no article)

Russian is easy.
Korean is easy.
Italian is easy. Chinese is easy.
j__anese is easy.
French is easy.

Generalizations**

Russians are friendly

Koreans are friendly.

New Zealanders are friendly. The Chinese are friendly.
Chinese people are friendly.
The j__anese are friendly.
j__anese people are friendly.
The French are friendly.
Frenchmen are friendly.

*If you are talking about a specific group (of Russians, etc.), the article the must be used.
Most of the Italians (in this class) are female.
Most of the j__anese (in the restaurant) are from Kyoto.
**If you are talking generally, no article or preposition is needed.
With s: Most Americans speak English. Not: Most of Americans
No s: Most Vietnamese live in Asia. Not: Most of Vietnamese

See also:
JustSomeLyrics
77
77.112
AREA & VAN GOGH DOZVOLI MI Lyrics
Nino Ferrer l'arche de Noé Lyrics