The Greek that is translated as “convulsions” or similar in English is translated in Low German idiomatically with the reduplicative rüttel un schüttel for “shake” (translation by Johannes Jessen, publ. 1933, republ. 2006).
Following are a number of back-translations of Mark 9:26:
Uma: “The child cried out, his body thrashed, and the evil-spirit came out. When the evil-spirit came out, the child no longer moved, he was like a dead person, with the result that many people said: ‘He’s dead!'” (Source: Uma Back Translation)
Yakan: “So-then the demon shouted/screamed and the child was very much convulsed by him. Then the demon came out from him. The child was like a dead person, that’s why most of the people said, ‘He is dead already.'” (Source: Yakan Back Translation)
Western Bukidnon Manobo: “And then that demon caused the child to cry out and made him convulse and then left him. And the child was as if he were dead because he didn’t move. And the people were saying that he was dead.” (Source: Western Bukidnon Manobo Back Translation)
Kankanaey: “The evil-spirit, it convulsed him extremely while-simultaneously he made-him-scream, then he suddenly-left-him. The child’s appearance was like a dead-person, so the many-people thought he had died.” (Source: Kankanaey Back Translation)
Tagbanwa: “That evil spirit cried out. Before he left him, he caused him to convulse/stiffen-in-spasm very much, that you could say that child was dead. That’s why many said, ‘Well, expletive, he’s dead now.'” (Source: Tagbanwa Back Translation)