Does anyone know if it is possible (and how) to add Thai language to a Samsung Tab.

On my Galaxy Note I can change the language in the settings menu, under "language".

Sure...just change the language setting as already recommended.

If it is an Asian version... Else visit Samsung Service Center.

If you don't have an Asian version its very difficult to do but you can always install man man Thai keyboard from Google play and at least be able to type.

Depending what you want to do. If you only need a keyboard and have a non Asian device, just download as advised above.

To change the language for a non SE Asia device, i.e. get menus and notifications in Thai, you have to download the official Samsung ROM for your device covering the area you need, and flash it on your Tab. That will reset your device to what effectively is the SE Asia edition with local languages for the area.

There are plenty of descriptions available on the Internet. It's not especially difficult, but can be quite intimidating if you're not into your geeky mode.

I've done it several times on my older devices that went into inheritance with my Thai family.