[NTG-context] Better support for Croatian in lang-sla.tex (patch included)

Hi, I made some modifications to lang-sla.tex that improve support for Croatian language (quotes, quotations and translations). Hope this can be included in ConTeXt. Regarding translations, I assume existing were done by Mojca, and they are certainly much better than Slovenian tranlsations that I would make :-) -- Vedran Miletić