I don't think we'll add Cyrillic mapping - it just gets too much, we can't implement full utf-8 mapping in a translator file. If you want cyrillic characters you should use a utf-8 capable bibtex implementation.

Even with that, though, I believe the current citekey generation function skips everything that's not ascii. That's pretty easy to disable in a custom version Currently
https://github.com/zotero/translators/blob/master/BibTeX.js#L2125
lists the _allowed_ characters. That could easily be turned into a version only banning certain characters (likely # % ' ( ) , = { } ~ and backslash )

I don't know enough about bibtex implementations - I don't really like the idea of adding yet another option to the dialog, but it would be easy to use utf-8 citekeys for utf-8 bibtex export.
Someone more knowledgeable would have to chime in on whether that makes sense (and then there's always the problem that this will break existing citekeys).

Yes, I expect manually specifiable citekeys for Zotero 4.2

please don't double post. Answered here
https://forums.zotero.org/discussion/23785/parse-markup-tags-in-titles/#Item_8

I use biber backend program, not Bibtex, I meant the bibtex file format after being exported from Zotero, please look at this workflow of bibliography citation in LaTeX using Zotero + biblatex package + biber backend + text editor http://tex.stackexchange.com/questions/18848/workflow-for-managing-references/114973#114973

I wish if there is something like this:
- dedicated shortcut key in Zotero to copy the citation key in bibtex format, but only citation key devoid from any surrounded mark-up strings
- parsing properly the italics, even if it is repeated twice in the same title of Zotero entry

- you can have a look at the workflow and suggest more improvements may be

@simifilm, biblatex package works well with bibtex format (I didn't experience a problem yet), the nice thing about biblatex you can customize styles as you wish from the bibtex format, whereas the biblatex translator of Zotero now is very buggy (the date field is horrible), until we have a decent one, then we can talk about biblatex format in Zotero, till now the bibtex translator is suffering from these issues:
- citation key author_title_year, is not a good idea IMHO, the title if the title would be marked up with some tags then you will see these tags flowing into your citation key, which is not an elegant thing, citation keys should be stable, immutable, and informative as much as possible
- parsing of the translator is not working if I do rich text tagging in the title more than once, if I am a chemist, or biologist, I would do this frequently with species names, chemicals, etc..., then this would be a nightmare to see the first tag parsed correctly, the others are not and will make it into your .bib file
- I would expect a dedicated shortcut key to be assigned in Zotero bibtex translator to copy the citation key ALONE (without any mark-up strings or any other fields) of the selected reference(s) into clipboard to be pasted in the tex editor (saves a lot of time), like CTRL+ALT+B, is not bad.

Hello,
There is a simple way to (temporarily) fix this for Zotero 4.0:
in %ZOTERO_DIR%/translators/BibLaTeX.js replace line 235:
var citeKeyCleanRe = /[^a-z0-9\!\$\&\*\+\-\.\/\:\;\<\>\?\[\]\^\_\`\|]+/g;
with:
var citeKeyCleanRe = /["#%'(),={}~]+/g;

Also, for citation, you may use script from here, - save it under %ZOTERO_DIR%/translators/BibLaTeX-autocite.js, and enable in Settings. It will copy links in the format \autocite{%author%_%title%_%year%}, compatible with the fix shown above.
And if you want to copy just key, without any tags, replace line 228 from
Zotero.write((first ? "\\autocite{" : ",") + citekey);
to
Zotero.write((first ? "" : ",") + citekey);
and remove line 233.