Do Japanese People Understand The English Words Written On Their T-Shirts?

The idea of (mostly Japanese) people wearing t-shirts that say something ridiculous in English, which the wearer doesn't seem to understand, leaves the internet community tickled pink.

We assume the people in the pics have no idea what their shirt says, which leaves us ROFLing hard when their shirt says something like "I'm about to let one rip" or "NAKED", but are we wrong? Do they actually know what their shirt says? Let's find out!

(YouTube Link)

This video from Fuji Television shared by NeKo JGT shows that many Japanese people wear tees with English words on them because they look cool and not because they know, or care, what the tee actually says.
So English tees are basically the equivalent of those Chinese character tattoos people love to get without actually knowing what they mean, only the tees can be easily removed...