Chinese and Japanese text do not work, but other scripts seem to be allowed:
Cyrillic (Russian) text: В старину борщом называли похлёбку из борщевика.
Georgian: ბორში ჭარხლით, რომლისგანაც ის დამახასიათებელ ფერს იღებს.
Hebrew: קיימים הרבה מאוד סוגים של בורשט.
Arabic: اقلیت ارمنی مقیم ایران در وارد کردن این غذا به سفره ایرانی سهم عمدهای داشتند.
Hindi: विभिन्नवर्णानां पालङ्का
Bengali: ক্রিমের সাথে বোর্শ।
Korean: 원래는 데워서 제공되지만, 여름에는 시원하게 제공되기도 한다.
(These text samples are sentences from the Wikipedia articles about borscht.)
It is interesting to note that there are SE sites about Chinese and Japanese language, which do not seem to have a problem.
As pointed out in @Michael E2's link to this Meta.SO thread, it appears to be deliberate. Posts containing CJK characters are not allowed for the time being because of spam that contains these characters. Apparently no consideration was given to any legitimate use of the same characters.
This is without a doubt the most idiotic spam-fighting method I have ever seen. A more reasonable approach would have been to have first posts be invisible below a certain points threshold until they have passed through the first-post review queue. When we saw a rising tide of spam on a forum I was involved with in the past, we did something very similar to this, and it solved the problem overnight.