Yes, I speak from experience. How long and for what, is irrelevant. The smaller he can speak and engage with the Thai prisoners the more left alone he will be, as long as he follow the prison rules and show common respect for other inmates. I could speak Thai also at that time, now 20 years ago, and that made it easier for me, but also getting more demands from Thai prisoners as they saw it as I could understand both the language and the system.