Hi Marie Claire,
You can find more information on the spell-checking options here - enabling/disabling spell checking and using custom dictionaries. You can find .TDF dictionaries in this forum post and if the French dictionary does not seem to be sufficient for the needs of your application, you can create your own as described in this blogpost.
Try it out and let us know if you need further assistance.
All the best,
the Telerik team
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