Well, i come from a Debian enviroment and yes, it is possible.
READ what I post completely before doing anything, please.
One thing, when installing a Linux Distro, It ask you the language to be installed, that information is used by locale to use the proper language on the enviroment and packages. I do not know if bt5 has the choice to pick a certain language. I have installed it many times but in english and I do not remember seeing a language selection question when installing.
As far as I know, it is only in English. But if you want to give it a try you can do this, which as I said, is for debian enviroment:
1. Find the /etc/environment and make a backup of it (just in case): Once in /etc: mv environment /etc/environment.backup. The original one has a "PATH=/usr/bin/...bla bla bla"..
2. Then create a new "environment" file and with one line: LANG="xx_XX.UTF-8" where xx_XX should be the French language (fr_FR, I do not know it, but for spanish is es_MX, es_ES, es_AR, etc) save and exit.
3. Then just reconfigure the locales package: dpkg-reconfigure locales and follow on screen instructions. Always use unicode (UTF-8) when you are asked.
4. Once generated, just reboot..
Do it under your own risk, again, in Debian stable, testing and sid is the way to do it..
You will only change the language of environment, not packages (Firefox, Iceweasel, etc), in such case you might want to use the language plugin.
There is another way, but try this one first, because the other one is a debian-way too, and need to install some app (perhaps not in BT5 repos), and this one will make the language change even to packages, if they are available in the distro's repos..If they are not, you will have to stick to the english version...