speed on this devices is limited by processor power, not much depending on FW version. For the 6" series the highlighting and marking functions were improved by every new version, I assume this is true for the 9" as well.
What exactly do you define as language support? In /menu/language my 602 has about 40 different languages for the UI, dictionaries are available for nearly every living and some dead languages and with books it is only limited by fonts for non-latin alphabets.
If there are differences between the versions sold in different countries they are only minor, like different games or so, but you can improve performance by adding a whole bunch of 3rd party apps, just have a look at this thread
For the PRO series I always recommended the Coolreader as a near-perfect reading app (although only for non-DRM books), maybe it is worthto give it a shot on the 912 as well.