Could be a mark or sign that identifies the one who did this to him. The one writing wouldn't need to understand what it meant.
Could be a curse against them, in whatever language he felt had the most power.
Could be a prayer offered up to whatever spirit or deity he looks to, or one that reached out to him. Likely in a language he knows.
If he was trying to achieve a goal (e.g., he was also trying to reunite those who were scattered), it could be clues about how to carry that out, that he had not been able to do (e.g., a list of places the others might have been taken). I would think in that case he might try to write in a way that hides the meaning from those who tortured him, whether in a language he expects they wouldn't know, or in riddles or cultural references.