I'm pretty sure the T|X has flash ROM, but actually Palm OS 6.0 is NOT coming. First it was called 6, then it was called Cobalt, and then it was cancelled. This is old news by now. The next Palm OS will probably be based on Linux, but Palm is said to be considering writing their own OS or switching to Windows Mobile. I don't think anyone knows what's coming next at this point. Palm almost never makes major OS upgrades available to existing users (and lately, not even minor upgrades).
Some Palms have multi-language support, and some don't. Usually, if you buy it in Europe, you will get the EFIGS version (English, French, Italian, German, and Spanish), but the US versions usually have only one language.
The great thing about standards is that there are so many to choose from. So I don't know what you mean by "standard bluetooth GPS," but I suspect there is a Bluetooth GPS unit out there somewhere that works with the TX and conforms to some standard of some sort.
There is a new article where Palm's CEO states that he is dedicated to the Palm OS and will continue to support it for years. That still leaves the question of what the future of the Palm OS is, and whether future versions will be able to run existing programs. I hope that future versions will at least provide emulation if they cannot run existing programs natively, similarly to how OS 5 devices emulate pre-OS 5 programs.
But you can play the waiting game forever... almost all operating systems have required their developers to rewrite their programs for a new architecture at some point, and in fact most have done this several times (e.g. Mac OS->PowerPC->Mac OS X->Mac OS X86, or older versions of PocketPC -> Windows Mobile).