I have been checking to see exactly what's been added in terms of additional unicode characters. The feature description merely states 'Cyrillic, Chinese, Japanese, Korean' - but I'm finding it is much more, and somewhat less, than that.
I've seen reports that at least Chinese is not complete, and is missing some common glyphs, but am not qualified to check that (or Japanese or Korean), beyond verifying that indeed a great number of glyphs are supported. But as they say, almost isn't.
As to Cyrillic, I can confirm that it supports the basic Russian alphabet, and most other alphabets based on Cyrillic, as well as most 'extended' Cyrillic. However it is missing some glyphs needed for at least Macedonian, Bulgarian, Kildan Sami, and Yupik (reference: Cyrillic (Unicode block)
). The missing glyphs are as follows:
0400 CYRILLIC CAPITAL LETTER IE WITH GRAVE
040D CYRILLIC CAPITAL LETTER I WITH GRAVE
0450 CYRILLIC SMALL LETTER IE WITH GRAVE
045D CYRILLIC SMALL LETTER I WITH GRAVE
048A-048F, 04C5-04C6, 04C9-04CF, 04D4-04D5, 04DA-04DB, 04EA-04ED, 04F6-04FF
It also does not include 'historic' Cyrillic in the range 0460-0489.
But beyond Cyrillic, it fully supports Armenian, Thai, Lao, Myanmar, Georgian, Ethiopian, Cherokee, Canadian Syllabics, Ogham, Runic, Buhid, Khmer, Mongolian, Limbu, and Ol Chiki - as far as I can tell. (Thai probably isn't really supported, I doubt it's able to properly break lines, etc.) I'm not able to assess readability. I'm not sure if Latin and Greek were complete before, and didn't really check these much - didn't notice anything missing.
It also looks like the 3 typefaces - regular, condensed, san serif - all look the same for the non Latin scripts.
Another good thing is that the fonts used for the item lists, browser, etc., seem to have at least the same level of support (I suspect the browser is even more complete, but haven't checked this yet). So you can use the new characters for Title/Author etc., and I would suspect review text that uses non-latin scripts will display properly on Kindle now, though I didn't test that.
Sadly, no Klingon.
But I'm excited to finally be able to make some Russian+English ebooks that can be read on Kindle without font hacks or settling for PDF.