Languages simply don’t agree on how to split the usage of words. Or grammatical case. Or if, when and how to do agreement.
Just for the sake of example: how are they going to keep track of case in a way that doesn’t break Hindi, or Basque, or English, or Guarani? Or grammatical gender for a word like “milk”? (not even the Romance languages agree in it.) At a certain point, it gets simply easier to write the article in all those languages than to code something to make it for you.
I don't know what the WMF is planning here but what you're pointing out is precisely what abstraction would solve.
If you had an abstract way to represent a sentence, you would be independent of any one order or case or whatever other grammatical feature. In the end you obviously do need actual sentences with these features. To get these, you'd build a mechanism that would convert the abstract sentence representation into a concrete sentences for specific languages that is correctly constructed according to those specific languages' rules.
Same with gender. What you'd store would not be that e.g. some german sentence is talking about the feminine milk but rather that it's talking about the abstract concept of milk. How exactly that abstract concept is represented in words would then be up to individual languages to decide.
I have absolutely no idea whether what I'm talking about here would be practical to implement but it in theory it could work.
Of course you do. The beauty of abstraction is that these language-specific parts can be factored into generic language-specific components. The information you're actually trying to convey can be denoted without any language-specific parts or exceptions and that's the important part for Wikipedia's purpose of knowledge preservation and presentation.
For writing a story or prose, I agree.
For the purpose of writing Wikipedia articles, this specifically and explicitly does not matter very much. Wikipedia strives to have one unified way of writing within a language. Whether the "I" is masculine or not would be a parameter that would be applied to all text equally (assuming I-narrator was the standard on Wikipedia).
If your article talks about the concept of a living pig in some way and in the context of that article, it doesn't matter whether the flesh is included, then you simply use the default word/phrase that the language uses to convey the concept of a pig.
If it did matter, you'd explicitly describe the concept of "a living pig with its flesh" instead of the more generic concept of a living pig. If that happened to be the default of the target language or the target language didn't differentiate between the two concepts, both concepts would turn into the same terms in that specific language.
The same applies to your example of the different forms of "I" in Japanese. To create an appropriate Japanese "rendering" of an abstract sentence, you'd use the abstract concept of "a nerdy shy kid refers to itself" as the i.e. the subject. The Japanese language "renderer" would turn that into a sentence like ”僕は。。。” while the English "renderer" would simply produce "I ...".
Yes, of course they would have to do that. The cool thing is that this it'd only have to be done once in a generic manner and from that point on you could use that definition to "render" any abstract article into any language you like.
You must also keep in mind that this effort has to be measured relative to the alternatives. In this case, the alternative is to translate each and every article and all changes done to them into every available language. At the scale of Wikipedia, that is not an easy task and it's been made clear that that's simply not happening.
(Okay, another alternative would be to remain on the status quo with its divergent versions of what are supposed to be the same articles containing the same information.)