Why do we say English language has only one alphabet if English words (and as an extension sentences and phrases as well) can be written in alphabets of many other languages?
EDIT Adding the requested clarification to the question. Letters from alphabets of other languages can be strung together to form similar sounding English words. In essence, transliterated versions of Keats or Frost can be read (and understood) in any language provided the reader knows English.
At the querent’s request, I’ve taken my comments above and converted them to an answer, though I’m not convinced they should be:
A given language is conventionally written in only one alphabet* by its native speakers. This establishes the context for saying that it is “the alphabet” for the language. If another writing system has ways to express the same sounds, one can transliterate English–or any other language–into that language’s writing system; that doesn’t mean that the transliterated language thus has that writing system as an alphabet.
* There are exceptions: Japanese uses kanji and two sets of kana (three writing systems), Korean uses hangul and hanzi (two writing systems), Chinese uses ideographs and a phonetic script called Bopomofo (two writing systems) and there is at least one Slavic language that is written with either the Cyrillic or Roman/Latin alphabet depending on where the speaker is. The point here, though, is that transliteration does not mean that a language “has as an alphabet” the writing system used for the transliteration.
A statement like "the English language has one alphabet" is just a vague description of cultural convention. There is no special deep or rigorous meaning to it that prevents English words from being written in other ways.
Likewise, if I say something like "the word treat has one spelling, T R E A T" it doesn't mean "it is physically impossible to use any other spelling" or "it will be impossible for anyone to understand what you mean if you write the word treat as 'treet' ". It's just meant to convey an idea like "T R E A T is the only conventional spelling for treat in present-day English". Or to give a cultural analogy, when people say things like "The English speak English" they don't necessarily mean to make some kind of nationalistic, essentialistic statement about the true nature of being English: a statement like that is likely to just be a generalization meaning "the typical, or stereotypical, English person speaks English".
There is only one alphabet in which it is conventional to write English. There are many other languages that are only conventionally written in one particular alphabet. But there are also a few languages that have more than one conventional alphabet. For Turkish, it used to be conventional to use the Arabic alphabet, but it is currently conventional to use the Latin alphabet. Serbian is a rare example of a language where it is considered standard to use either the Latin or Cyrillic alphabet. And of course, many languages are written in the same alphabets. So the relationships between languages and alphabets are not necessarily one-to-one, one-to-many or many-to-one. They're arbitrary. Lots of things about writing are arbitrary.
