Let's try to break this down a bit. First:
We allow Italians and Germans because these words are both adjectives and nouns.
1) In what respect would you consider
Italians an adjective, having a plural 's' as it does? I take it you mean
Italian?
2) Are you really saying that
Italian is
both a noun
and an adjective in form? What is it about the form that determines this? If you're looking at the morphology, it has an adjective suffix. But what makes it formally a noun? The fact that it can transform into the plural form
Italians? Or the fact that it can be preceded by a determiner, for example? If I refer to your new shirt as "
very you", what word class is
you?
3) If you mean that the word belongs independently to two classes irrespective of an instance of language in context, does this mean that even in the phrase
Italian brogues, the word
Italian is still both a noun and an adjective?
4) Or do you mean that the word
can be either an adjective
or a noun? Then wouldn't that depend on its use/function in a phrase, not on the form? If not, then what would determine the class?
Incidentally, Chinese as the name of a language is a noun.
Always? How about in the phrase
Chinese characters, where the word
Chinese clearly refers to the language. How about
English in
the English language?
Look at these differences between the a and the b/c sentences. :
1a. My sister is going out with a German/an Italian.
1b. *My sister is going out with a Chinese/a Dutch.
1c. *My sister is going out with a wealthy/rich.
2a. Germans/Italians are very friendly.
2b. *Chineses/Dutches are very friendly.
2c, *Wealthy/Rich are very friendly
3a. *The German/Italian are very friendly.
3b. The Chinese/Dutch are very friendly.
3c. The wealthy/rich are very friendly.
4a. Let's hear a German's/an Italian's views on this.
4b. *Let's hear a Chinese's/A Dutch's views on this.
4c. *Let's hear a wealthy's/rich's views on this.
Sorry, I don't get the point here. Of course, I understand all that.
There is no obvious phonological reason for not having *Swisses, *Welshes, *Dutches, *Chineses.
To me, it is obvious. Why else would it be (mostly) only those words ending in certain sounds that are irregular in this way? I don't know the exact historical explanation off-hand but I'm confident it's phonological. I could try to find out. So when we say
The Chinese are coming, this is a semantically equivalent statement to
The Italians are coming, in that we conceive in both a group of people, so where's the grammatical difference?
And it has become quite common to speak of Englishes when we are speaking of types of language.
I don't see the connection. Of course I'm not suggesting that you can't pluralise words ending in
-sh. Englishes means varieties of English. In the same way, you could say
Chineses, referring to varieties of Chinese.