I went to a presentation by a company that dealt with translation, he mentioned this issue and his recommendation was to simply not try to be smart with it and have separate strings where pluralisation is done properly in each one.
What about gettext _N? I have used a library with a similar interface, and we didn't even get complaints for Slavic languages with very complex pluralisation rules.
That works when there are a small number of possible numerical values. But cldr has a table of plural rules for each of these languages and it's not bad to solve the problem for arbitrary integers.
