*First thing you should know is the language in Japan they speak Japanese that will really come in heady when selling t shirt to Japan.
*Second thing you should know is there religion you need to know there religion because you don't what to send millions of produces to the country that no one beliefs in. But in Japan they have many beliefs.
*Third thing you should know is what music,movies,television shows or any other things they like in that country.
* Forth thing you should know is what team they have like in soccer,sumo wrestling,ice skating and more you can use those as a t shirt logo too.
*Fifth thing you should know is what kinds of foods they eat in Japan for example they eat: sushi,sashimi,tempura and more you can also use some of those as shirt logos.