Why do we say but 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.

The Chinese are here.* The Japanese 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. Japanese 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 Japanese are friendly. Japanese 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 Japanese (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