Sticking to ASCII in this day and age is nice and fun if English is your mother tongue, but it's a big middle finger to the rest of the world. Please don't.
You can use Unicode characters in item descriptions, so you can write these texts in Japanese, Finnish, or Greek. Only the “syntactical elements” (like checkboxes, priority, due dates) are made up from ASCII characters, to ensure that they are easy to type.
This run-time-option-only approach would be much better than the "be liberal in what you accept and strict in what you emit" philosophy that tends to cause fragmentation on lots of different levels.
Please don't. The equivalent in programming languages would be localizing the syntactic elements ({}, (), [], '.', etc). Its much simpler for a language ecosystem to have the same syntax regardless of the programmer's language.
As far as I know, international keyboards can still type these characters just fine.
There are languages written from right to left or top to bottom. No standard I know of are supporting such flexibility in syntax. And it shouldnt be necessary - if the <user text> items within the grammar support unicode but the keywords are ASCII only it can MORE easily adapted than supporting unicode....
"mother or preferred"
-- I don't disagree with your basic point, but I like to be included :). There's a Lot of us around whose native / mother tongue is not English, but we prefer it for computing tasks (and frequently gaming / movies / entertainment too - Forums are littered with folks complaining of having to "enjoy" a horrible localized version of something, instead of the original English)