#16: Liturgisch ist das der richtige Begriff.
only men can be acolytes in the Roman Catholic Church:
Is there a distinction between acolytes and altar servers? If so, please explain.
There is a distinction between the two, and the office of acolyte is reserved to men alone. Canon 230 §1 says: "Lay men whose age and talents meet the requirements prescribed by decree of the bishops’ conference can be given the stable ministry of lector and of acolyte, through the prescribed liturgical rite . . . "
The source linked by A_monkey is apparently a missal from 1970. I would be surprised if that were as informative as a more recent source about current usage.
#12: Meine Vermutung ist, dass der Begriff "acolyte" von nicht-kirchlichen Menschen und von Leuten aus anderen religiösen Backgrounds eher nicht verstanden wird und "altar server" (oder auch "altar boy/girl") allgemeinverständlicher ist - was meint ihr dazu?
I don't think it matters here what people from non-religious backgrounds or other religious backgrounds do or don't understand. In my opinion, your applicants should write the term commonly used in the Catholic Church in the target country.
Imagine families reading the application. They see the word altar server and react in one of a limited number of ways which depend on their background: "Oh, look, it says here that Susanne was the leader of a group of young altar servers at her church."
Scenario 1 (non-religious family, of which there are fewer in the US than in Germany):
A: She must like kids and she has experience.
B: She sounds very religious and we're not. Let's look for someone else.
Scenario 2 (religious family from a church that doesn't use altar servers or calls them something different)
A: Altar server. I wonder what that is. Let's find out. (Then B or C.)
B: I think that is what what we call an acolyte. That's nice. She's involved in her church and likes kids.
C: She's Catholic. I think I'd rather find someone who is Jewish/Muslim/Protestant...
Scenario 3 (Catholic family)
A: Susanne would probably fit right in with our family. It would be nice to have someone who wants to go to Mass with us.
B. She must like kids and she has experience.
C: She sounds very religious and we're not. It might be awkward that we never go to Mass. Let's look for someone else.
In my opinion, applicants who are religiously active are not going to be viewed negatively by most families in the US. Families who are religiously active may prefer someone who is similar to them or they may not care that much. I am speculating, but I would guess that most would still probably rather have someone who is religious, even if they are of a different faith, than someone who is not religious at all. Some families may view the information negatively, but mostly families are just trying to find applicants who will be a good fit, and information generally helps in the decision process.