A judge is expected to decide Friday whether to formally detain or release Paul Watson, the Sea Shepherd Conservation Society said.
The ruling was originally due Wednesday but was postponed, and Thursday is a holiday in Germany, spokesman Peter Hammarstedt said.
But there were some developments in the case Wednesday, with the public prosecutor in Frankfurt officially requesting a preliminary extradition arrest warrant for Watson.
The warrant would allow authorities to hold him for 90 days pending extradition.
Hammarstedt visited Watson in jail, where he is being temporarily held in isolation.
He said the Toronto-born activist is "keeping his spirits up" but fears for his safety should the court rule against him.
Watson is "absolutely certain he wouldn't be given a fair trial in Costa Rica," Hammarstedt said.
In a statement issued from his cell, Watson said the group has "made some powerful enemies" in its efforts to defend the lives of whales, sharks, seals and other sea life.
"Therefore the question must be asked why Germany is now taking into account accusations made by illegal poachers," he said.
Watson is wanted in Costa Rica for allegedly endangering a fishing boat while filming a documentary in 2002.
He was arrested Saturday at Frankfurt Airport on an international arrest warrant issued by the Central American country.
Sea Shepherd has maintained his arrest is politically motivated. The conservation group is calling on German officials to dismiss the case.
The group said the minister of justice has the power to halt the proceedings on political grounds _ a fact it said the prosecutor recognized in his motion.