Just a quick note to say that it sounds like a great trip! I think that the first port of call would be to start looking at the EAIP documents for the countries you will be visiting. I cannot link to all of them but here's two to get started:
Netherlands:
eAIS Package for NETHERLANDS UK:
NATS UK | AIP They will not all be online (Germany is always hard to find) but this should give you an appreciation of what is needed to fly into the country.
ATC will be able to use English, although you will also encounter some other languages no doubt.