The lawsuit is pretty insane ... 48-year-old Damian Chapa (who also starred as Ken in the "Street Fighter" movie) claims he met Roxanna Foell in Germany in 2006 at the Berlin Film Festival.

According to the lawsuit, the two hit it off, traveling around Europe together for several months -- enjoying a "very passionate" and "consensual" sexual relationship -- before moving to Damian's California apartment later that year.

Around that time, Damian says he learned Roxanna was pregnant with his child, but when he asked her to marry him ... she dropped a BOMB, confessing she was ALREADY MARRIED to a dude in Germany.

According to the suit, Roxanna swore she'd divorce her husband -- but when it sunk in that Damian wasn't a rich Hollywood celeb, she reconciled with her hubby and move back to Germany.

Damian says his son was born in 2007, and Roxanna made it HELL for him to see the boy.

Damian claims he filed a paternity suit to get visitation, and Roxanna reacted by telling German police he had RAPED her -- so when Damian arrived in Europe to visit his son, he was placed under arrest. He claims he was in custody for 7 WEEKS.