Many thanks, Yutaka. I can confirm that the Bulgarian spell checking is working fine now. I will try later on with Russian, but do expect that it will be all right, as well.
So far as the traditional spell-checking facilities I suggested in an earlier posting are concerned, these would be very useful if one has to modify an existing large file, especially if has not been spell-checked before and there is intensive syntax highlighting. In this case it can prove difficult to spot all underlined words. The “on-the-fly” approach works very well when creating a new file.