Thank you Claudio, all understood.
Claudio Beccari wrote:
> Your XeLaTeX MWE has \hyphenation in the wrong place. The explanation is
> that the \hyphenation exceptions introduced with command \hyphenation are
> connected with the current language. The russian language, the only one
> specified after polyglossia, becomes active only after \begin{document};
> before that the default language is language 0, that is english.
Philip Taylor
