English spelling works fine. Installed second language (Russian - aspell-ru), enabled language autodetection. Spelling itself still works fine - it detects both languages correctly and underlines misspelled words in both but suggestions for misspelled words work only for English, in Russian it shows just some Latin letters. Any ideas? Kate 17.12.3 - should be ancient (ubuntu 18.04).

How about updating your installation to something more recent and see if this still happens? Ubuntu 18.04 will reach its EOL soon anyway