Fair that I should have said they have two phonetic writing systems, decidedly not alphabets. I'm not sure the distinction is one that matters for what we are covering here?
It's a feature linked to spoken languages, since it is a feature of the relation of non-spoken (usually written) language to a spoken language.
But it is not a feature of a spoken language.
> Sign language, for example, is not phonetic, as many users of it cannot speak or hear.
Yes, in causal terms, the fact many users of sign languages aren't familiar with the sounds of the spoken language is a reason sign languages tend not be phonetic, but they are not phonetic in definitional terms because the symbols in the sign language do not represent the sounds of spoken language.
But it would make no sense to call a spoken language phonetic (except maybe if it was a code for a different spoken language, in which the phonemes in one mapped to the individual phonemes, rather than ideas, of the other.)
I get what you are aiming at, but phonetics is about speech. Is why you can reliably say how many phonemes different languages have. If you had to cover all vocalizations that people could do, you would have a bit more trouble.
I'm largely comfortable with the idea that there is something lacking in the orthography of English. Fully comfortable, even. I'm growing frustrated with how many are pushing the idea that it is not phonetic. The system is literally to convey, in writing, the words that you would speak in English. And the word "phonetic" captures that perfectly.
If you want to argue that we are building a new use of the word "phonetic" applied to writing that supersedes "orthography" and related terms. You do you. It still seems nonsensical to me and only works if you ignore that we have an alphabet that is literally used to convey speech sounds.
> Of course spoken Japanese is phonetic
"Phonetic" is not a feature of spoken language, but of the relation between other language forms (usually, written, but you could make the same distinction for, say, sign languages) and spoken language.
> They even have two fully regular alphabets
I assume from "two fully regular" you are referring to hiragana and katakana, but those are syllabaries, not alphabets. (Romaji is an alphabetic system, though, but I don't know where you'd find a second one.)