We can say "They are Americans" but "They are English people".
Can we say "They are Englishes"?
When you say They are Americans you are using American as a noun. The word American can be a noun or an adjective. In both cases the word refers to a person from North America or to a US citizen. The letter s is added to American when it is used in the plural (this is the rule for most nouns in English unless they have an irregular plural).
The word English, when talking about a person originating from England, can only be used as an adjective and in English adjectives are invariable (they do not change in the plural).
English exists as a noun but not to designate someone from England. It can designate the language:
So if you say:
They are Englishes.
people might possibly understand you are talking about varieties of the English language but you cannot use it to talk about people's nationality.
The noun english can also designate a spinning movement of the ball in bowling (wiktionary). In that case it is not capitalized.
Also note that several dictionaries define the word "Englisher" as an "Englishman" or "English person" - so you could say "They are Englishers".
To add to Laure's answer:
adjective : American - English
noun (sing): American - Englishman / Englishwoman
noun (pl) : Americans - Englishmen / Englishwomen
Of course, adjectives don't have plural form (in English language).
The following construction seems correct, but sounds wrong.
For the people groups we were discussing before (the Britons and the Saxons), they are Englishes.
This works by forcing what is normally a group noun to be a single noun (by talking directly about the group) and applying another plural operation by talking about more groups of its kind.
I see so many questions that go by here where it turns out that the proposed verbage ends up meaning something, but not what was intended. Here is another case. It is rather difficult to construct something short with correct grammar but no meaning.