It is very easy to change languages in Linux. If you are using GNOME, you could open a terminal, and then change the LANG variable to the language you prefer to use. Then, you just run the program from the terminal and you'll be running that program in the language you just selected. Because of this you can run many programs in different languages at the same time.
The thing is, if you didn't install the "locales" (or, the languages your computer can use to display for program/time/etc.) for other languages, as seems to be the case, you won't be able to do this on regular programs (on programs that use their own language package this doesn't apply. e.g. OpenOffice, firefox, etc). So:
This shows you the languages you can use in your computer.
If you don't have the locales needed, you will have to generate them.
In Debian the file that has all the files that can be generated is in /usr/share/i18n/SUPPORTED , in your computer it probably is the same, or a similar one (check
for more information).
to see what language you want. You said you want French, right? That would be :
You might want them all.
Now that we know what locale we want to generate, the next thing we have to do is exactly that, to generate them.
So, again in Debian, the name of the file is /etc/locale.gen (if you don't have this file you might want to see the man page for locale.gen <man locale.gen> and see where it is), you edit fr_FR.UTF-8 UTF-8, etc, all the languages you want to add(You better not forget English though...).
And finally, as root, you do
which will generate the locales, thus finally enabling you to use your new languages.
As I told you, all you need to do now in order to change the language of a program you are going to use is to open a terminal, change the LANG variable to the locale you'd like to use (for french in this case
) and the you just type the name of the program in your terminal.
BTW, in KDE you can do pretty much anything by only clicking around, you might prefer this desktop environment.