Ok, as a VATSIM controller, here are some replies:
- I cannot speak for our esteemed British colleagues, but yea we come in all flavors (or flavours if you will). Microphone quality varies, connection speed, accent and fluency. Often you won't be able to understand a controller. What to do? "Say again for <callsign>" and then "say again" again. If you feel it is the sound quality, just say "receiving you 2/5, please send text too" or something to that effect. If you just can't understand the accent or the controller is speaking too fast say "please repeat slower for <callsign>" or even ask them to alphabetize (for example to say Echo Golf Kilo Kilo instead of "Ga'wi' In'nationa'" 
If a controller tells you "are you deaf" tell them you can't understand them and there is no call for being a smartass. Then if it can be resolved, fine. If not, please do us all a favor and report them to their VACC (or ARTCC) supervisor. I can't stress this enough. Controllers are people and they can be having a bad day I suppose (although they should not be online then - it's a hobby, it's about having fun), but don't think that they are some kind of "guild" that always sticks together. We do weed out the bad apples (not that there are many).
- The universal language of ATC is English. That having been said it is ok to use regional language (real controllers do it too). My rule is (well, actually the Hellenic vACC rule):
1) If there is only Greek traffic on my frequency, use greek.
2) If *anyone* (Greek or otherwise) calls me in English, respond in English.
3) If there are *any* english speakers on the frequency, call everyone in English.
4) Small exception: If a person does not speak English well, we're not there to torture them, we continue in greek and advise other traffic of important instructions.
As for your first question, I don't have the PMDG birds, but I have the Wilco/FeelThere 737, which is just as complex. I would say about 20 minutes for the preflight, all the way up to engine startup. Clearances come quickly in LGGG, but I have been asked to wait on occasion in busy places like Heathrow or Atlanta.
Hope all this helps. Don't hesitate to ask anything. VATSIM rules!