What do you think Almog Meir Jan did during his 8 months as a hostage of Hamas?  He learned two languages of course.  Arabic from watching Al Jazeera, and Russian from speaking with his fellow captive Andrey Kozlov.  He also had been able to keep a journal while in captivity.

https://worldisraelnews.com/rescued-hostage-learned-arabic-and-russian-in-captivity/