One of my biggest pet peeves is something that is highly damaging to missionary work in Europe. It can manifest it in many different ways: here are two examples I've heard given at church.
1. A statement made in a Sunday School class for teenagers close to missionary age. "All of the good blood in Europe has already been converted, and they immigrated over here, and that's why missionaries in Europe don't get any baptisms."
2. A statement made by a church leader over the pulpit. "We're in Europe not for the Europeans, but for people from Turkey and other countries who immigrate to Europe."
BS. (I thought about spelling BS out, since it's certainly deserved here, but I'll restrain myself).
In the strongest sense possible, BS.
In response to the first statement: missionaries in Europe do get baptisms. They baptize actual Europeans. There are plenty of good Europeans in Europe, and some of them are willing to accept the gospel. Telling future missionaries they won't have success can be (and often is) a self-fulfilling prophecy. It's something Satan himself would say.
In response to the second statement: again, Europeans do get baptized. And once they get baptized, many of them remain active. Contrast that with non-Europeans who (in my experience) all go inactive. I served in two wards and two branches, and never once met an active member who was not either European or American (including South American). We tried working with a couple of these inactive members (Middle Eastern and African, mainly), but the language and/or cultural barriers were just too great--they didn't feel comfortable in the congregation. I know some missionaries in Europe in the past specifically sought out foreigners for easy baptisms, but such work is less effective. It doesn't work. It's not why the Lord sends missionaries to Europe. Missionaries are in Europe to serve Europeans.
If you know someone who's serving or who will serve in Europe, tell them this: Europeans are good people. They are intelligent, and they have good hearts. And missionary success among Europeans is very possible, as long as the missionary believes it's possible. Satan has his lies, and too often church members pass them on. Don't believe them. Believe in the people you're sent to serve, and believe in yourself.