Support for non-english keyboards

[JD]

Hi All,
I would like to try to type in none-english script.
I can either buy a keyboard made for a specific
language or buy a thin silicon overlay whichare made
for specific brand name keyboards, like HP,or Apple, ...etc.
Unfortunately, I do not find any overlays for generic USB based
keyboards.

My question is: what do I have to do so that the application
(such as FireFox, or Thunderbird, or even the gnome-terminal)
will display the script of that specific language?

Thanks for your help.