Android's Droid fonts allready support almost all languages. At least, if you change your location\laanguage preference in setup menu, and after this menu shown correctly in your language, then Droid fonts contain all needed characters set.

BUT if you talking about e-ink epubs, than that's another story. EE developers told me that they didn't include an option to load custom fonts or use custom css (and that's a little confuse me 'cos it's default option in adobe reader mobile). So I didn't spend many time to find out if there's an hack to load custom fonts, but anyway, you much likely need to look at esi/etc/fonts.conf , /system/esi/bin/ e.t.c.
But on the other hand, one of developer of "adobe reader mobile" told me that it's unlikely that I'll be able to add custom fonts if EE doesn't compile this option. But you'll never know until try it.