Considering how quickly the language moves to keep up with internet culture and new newsworthy names, new parlances, new memes, an IME has to do the equivalent of staying up to date with the equivalent of urbandictionary for users to be able to invoke the latest "lit" colloquialism. This is a full-time job on its own.
Imagine typing English by speaking. You can say "flower", but that might get written out as "flour". Some intelligence has to be implemented that picks the right one based on context, or give you the ability to correct it. That is where the complexity in east asian input methods come from.
(Yes, as English-speaking computer users we are very lucky. The exact sort of symbols that readers of English expect map 1:1 to our keyboards. Still kind of a pain on a phone, though!)
Chinese orthography is just as standardized as Japanese and hanzi/pinyin dictionaries are not harder to maintain than kanji/hiragana ones. Some people have trouble with sound distinctions in Standard Mandarin that don't exist in their speech and enter incorrect pinyin (e.g. z instead of j or zh), but that can be treated as a typo, the same as phonetic misspellings in other languages.
Support for dialectal variations is essentially nonexistent in mainstream IMEs. People who want to use varieties other than Standard Mandarin would have to use shape-based input (including methods that decompose each character into smaller parts, like Cangjie) or send a voice message. (There are projects to create IMEs for other Sinitic languages, like https://hanhngiox.net/ but almost nobody uses them.)
(Do any Japanese IMEs support non-standard dialects or even other Japonic languages?)
I'm not sure I understand your point. Without keeping up with the new memes, IME still let people type them (it's simply not as easy when IME does not auto-suggest the new combination, users can manually select each Chinese character).
Regarding "an informal ambiguous layer", are you implying there is something more fundamental/low-level than the Chinese characters used in communication? If so, what is that?
There are entire companies that exist to solve just this problem that is basically orthogonal to Signal’s purpose and mission. While it would be great for there to be a top tier Chinese IME from someone we trust, it’s by no means an easy task like most people are probably envisioning.
They already have some romanizations like Pinyin that are largely phonetic.
But take something like "mao". Without any accents to indicate the tone, that could be:
* 毛 - like a dozen distinct meanings
* 猫 - a few meanings, but mostly "cat"
* 冒 - half a dozen meanings
* 昴 - one meaning
* 懋 - a few meanings
* 帽 - a few meanings
* 貌 - a few meanings
* 牦 - one meaning
* 矛 - one meaning
* 铆 - a couple meanings
* 锚 - one meaning
* 贸 - one meaning
* 茂 - two meanings
To be honest, I got tired of compiling the list at this point. There's lots more.When someone uses a romanized keyboard to type "m", "a", "o" you've got like two dozen possible characters that becomes. If you're trying to figure out from context which one the person might be intending, you need to look at like 60 different possible meanings in context and figure out which characters are most appropriate. And that's given the previous several things they've entered have been narrowed down to one character, but likely still have several meanings.
A lot of (I'd dare say most?) Chinese people do use romanized input (the alternative I've seen is very slowly drawing out each individual character on the screen), but whether the keyboard sees you type "I have a cute... mao" and decides you want 'anchor' or 'cat' has a huge impact on day-to-day usability for people actually using it.
The written language is vastly more complicated than the spoken one as far as I can tell. A syllable that can have 60 meanings is relatively easy to figure out in context, but when written the meaning has to be made explicit. As a really basic example, "ta" is both he and she. So they just don't have masculine and feminine pronouns, right? Wrong. 他 is he, 她 is she. These are said exactly the same so even given accents to indicate tone they're romanized identically. But when written, the distinction is made.
And if you make a mistake somewhere in all of this?
Well, baba (爸爸) is dad. baba is also poop (㞎㞎).
Wo ai ni, baba. I love you, poop.