Sergey Biryukov

Allows to create users with usernames containing Cyrillic characters.

Allow Cyrillic Usernames

Allows users to register with Cyrillic usernames.

pbosakov

This plugin will automatically transliterate permalink URLs when you save a new page or post with a title in the Cyrillic script. 
Romanization presets are available for all the major languages using the Cyrillic script: Russian, Ukrainian, Kazakh, Belarussian, Serbian, Tajik, Bulgarian, Kyrgyz, Bosnian, Mongolian, and Macedonian. 
The plugin can also romanize permalinks for any pre-existing posts, categories, products or other items with a title written in Cyrillic. 
Only UTF-8 encoding is supported.

Cyrillic Permalinks

Automatically transliterates Cyrillic letters in permalinks to their Latin phonetic equivalent. Multi-language. Can convert pre-existing permalinks.

Allow Cyrillic Usernames vs Cyrillic Permalinks