Changing the language doesn't work (WMPU 2.9.2-BP

Hello all,
we're building a site with WPMU 2.9.2 and BP with the bp-magazine theme.
We want to translate WMPU & BP in Greek -- since no recent .mo is available -- but during our first test we discovered that we can't change the site's language. As a test, we tried the French translation included in 'Languages for WordPress MU' and it doesn't work either.
The procedure we followed is:

- upload in wp-content/languages/
- upload (from in wp-content/plugins/buddypress/bp-languages/
- set define ('WPLANG', 'fr_FR'); in wp-config.php
- select French as 'Default Language' in 'Site Admin' -> 'Options'

but the site remains in English, no matter what. Any suggestions?

Thanks in advance.