See my other comment below, but all the things you're talking about work because English has a phonetic alphabet.
You can break 'be with ye' into 'b', 'y', and 'e', and drop the rest. In languages like Chinese which do not have a phonetic writing system, you cannot drop individual sounds within a word/character in a rule-based way like contractions.
Emojis are not a phonetic system, so unless you created an emoji for "they're", separate from the emojis for "they" and "are", you wouldn't have it as a word.
Haven’t read it, but society is quite changed in The Diamond Age, I think. In the protagonist even speaking English? If not, we could assume it is sort of “translated” into English (in the sense that most fiction that doesn’t take place on modern day Earth is).
No, but languages that use glyphs that are monophonemic and non-phonetic (one-sound per-glyph/character) don't have contractions, which are a function of removing some component part of a word when combining them.
For instance, both Chinese and Japanese use kanji/hanzi, but Chinese is monophonemic, and does not have a phonetic alphabet that characters can be broken into (radicals aside, which are not related to sound). Japanese does (kana).
As a result, combining 2 characters in Chinese never changes the sound of some sub-portion of a character; it's either the whole sound that changes, or nothing. In Japanese, on the other hand, individual kana within a character can change (e.g. Rendaku), so for instance 'hito' (person) put twice in a row becomes hitobito instead of hitohito, because hito is comprised of 2 kana characters: 'hi' and 'to', and 'hi' becomes 'bi'.
Emoji have no subcomponent characters, so you either would need an emoji for a contraction word in addition to the source words, or, more realistically, you just wouldn't have them at all.
In Japanese, yes, because it has a phonetic alphabet. For example, "konnichiwa" is often shortened as "kon'chiwa".
In Chinese, there is shorthand slang, but not shortening of words in writing based on spoken sounds (since it's not phonetic).
So even if "wo shi" (I am) might be spoken more quickly, there's no way to write that shortened version out in Chinese. You will actually see Chinese speakers use Latin characters and Arabic numerals for phonetic shorthand, (e.g. '88' for "bye bye", because 8 is pronounced 'ba'/'bai'.
"Goodbye" = "God be with ye" for example
You don't think "will not" when you say "won't", "won't" is just a word you use with a meaning you understand long before you can write.