In Greek (and Russian) the guillemots are different than the default. So the NewCM fonts include a lookup cv04 for the Greek and cv03 for the Russian to switch to the proper glyphs.
I thought that if cv04 has metadata grek(dflt) and if I switch to Greek using the polyglossia command \textlang{greek}{\newcmgreekguillemots «»} where \newfontfamily\newcmgreekguillemots[CharacterVariant=4]{NewCM10-Book.otf} then the substitution will work. But it does not. If I add to metadata latn{dflt} then it works. But is this proper? Shouldn't it work only with grek{dlft} ? It is either my understanding wrong or polyglossia does not do the proper switching or xetex does not pickup the change of language properly. In my sources I have added latn{dflt} for both Greek and Russian to make the fonts work. So if a test is needed I have to send a custom font. Thanks, Antonis.
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