Unfortunately, Decker does not currently support Cyrillic.
Decker uses an internal byte encoding called DeckRoman, which supports a variety of diacritics and special letters for Latin-based alphabets, and provides a range of fonts with those characters.
It is possible to represent other alphabets using a custom font, but this does not automatically make Decker recognize natural text input in those alphabets.
For a more determined tinkerer, it is within the realm of possibility to fork Decker and modify the lookup tables it uses for converting between UTF-8 and DeckRoman, but this would require some programming experience.