Can’t reproduce this yet, but would like some more info.
– does this happen regardless of what text you place in the field, even if its just a few characters? (E.g. тест)
– does this happen when you place Russian text in other fields in the WP to Twitter settings, or just these?
– does the same thing happen with other multibyte characters, such as Greek? (E.g. δοκιμή)