if they did serve, would have served under the order of a foreign order.
I am not sure they did serve anyone.
According to Bernard of Clairvaux, the goal of the crusade was to battle the pagan Slavs "until such a time as, by God's help, they shall either be converted or deleted". However, the crusade failed to achieve the conversion of most of the Wends. The Saxons achieved largely token conversions at Dobin, as the Slavs returned to their pagan beliefs once the Christian armies dispersed; Albert of Pomerania explained, "If they had come to strengthen the Christian faith ... they should have done so by preaching, not by arms".
Sounds more like pillaging than converting, it didn't work not then anyway by the looks of it.
The campaigns started with the 1147 Wendish Crusade against the Polabian Slavs (or "Wends") of what is now northern and eastern Germany. The crusade occurred parallel to the Second Crusade to the Holy Land, and continued irregularly until the 16th century.
I believe this also includes Poland.