Hello,my problem is :

When i login on multiplayer server,my flag doesn't load and there's a toribash logo.

Reinstal the game, or get the steam version it hasn't any bugs.

Did you add in your email when you created your account?
If not, try adding one in your user cp then logout of your forum account then log back in again.

(Might be wrong tho but I'm pretty sure you need an email with your account for it to add a flag)

Still have the problem

The next problem might be that you have an unstable IP (it changes every time you use the internet). If you get a stable broadband connection, that should add a country flag in for you.

This isn't really a huge issue, as not having a flag isn't gamebreaking. It might simply be that Toribash simply doesn't have Thailand's flag in its files, so you get the logo instead of a flag.