Although his time riding in England came during the Elite League/Premier League era, surely Freddie Lindgren takes first prize for the foreign rider who spent the longest time at just one club?
Freddie spent 14 years at Wolves, 2003-2013 and then again 2015-2017, and only ever rode for Wolves in England.
I can't think of another one club foreign rider who comes close to Freddie's loyalty to Wolves.