That's a lot of questions but I'll try to answer a few of them at least.
1. No, the commands and voice files are in English.
2. A summary of key commands can be found in the User Guide, p. 142, located in your PF3\Help directory. In PF3 I'd suggest selecting the 'Auto Tune Initial Facility' option (Options #2 screen, Advanced Options and INI File Tweaks, COMs section) so PF3 will automatically tune the initial ATC facility when you connect. On the Options #1 screen I would suggest CP Mode 2, so PF3 will handle the COMs for you.
3. Again on the Options #2 screen, Advanced Options and INI File Tweaks, at right, ATC Log section, select 'Show log on connect'. When you connect PF3 to the flight simulator this will open a window that shows the most recent ATC communications with you.
4. Voice control is possible with use of voice recognition software. Many PF3 users use MCE for voice recognition, but this is not required and PF3 can be used with key commands.
5. You can fly IFR or VFR, according to your flight plan. PF3 expects you to follow your flight plan and hit your waypoints, but how you navigate is up to you. You can use VOR/NDB, GPS, FMC in your aircraft, autopilot, or whatever you like.
6. Yes you can use flight plans prepared with other flight planners. PF3 can read flight plans in pln or xml format. If you use a flight planner there should be a way to export the flight plan in an FSX format and PF3 should be able to use it when you load it. Be sure the filed cruise altitude is correct, as PF3 expects you to reach your filed cruise altitude.
Hopefully that will help you get started.