It’s hard to say much without more details.
I know for a fact that with Palm OS at least, the localized versions of the OS are just a thin layer of stuff on top of the real OS and don’t interfere with most applications (those that play nice with the Big5 character set at least).
There are PCMCIA cards that dangle out to D9 connectors - I’m not sure what Sub-D9 is but if it’s not the standard little VGA looking ones, you can likely just use an adaptor.
If you are dealing with some sort of iNote or something like that, all bets are off. At a guess they can be made to run English based code, but what applications would there be? Supposedly they are based on Palm’s platform, but I have never been eager to try and test the theory, despite the attractive prices.