Hi Gabriela,
My apologies for the previous thread being closed prematurely. I was trying to make sure your system was working properly rather than offering you a full solution, and it looks like Mark assumed your answer meant the problem was resolved. We’ll do our best to help get you a solution here.
From the tests on my own MSM install with forums, adding the following to one particular site’s index.php file caused that site to offer spellcheck in French for the forums:
$assign_to_config['spellcheck_language_code'] = 'fr';
The other sites continued offering spellcheck in English. So it looks like this is indeed the proper way to do it. The real question now is: why isn’t your installation handling this correctly?
Could you tell me more about the directory and file structure of your MSM setup? The French site is its own MSM site, correct? And if that site has its own web root with its own index.php in it, adding the above to that particular site’s index.php should perform as expected. Just to be clear, you’re not adding the $assign_to_config variables to system/index.php are you?