Hi Marlene,
basically visforms uses the default mechanism of Joomla! for it's translations. Usually you would have installed the the French language on your website and set it as default for frontend and backend. When you have done this and made no further settings visforms should use the french translations by default. But of course you have to have the french translations otherwise the component would fall back on the default which is English.
There are different options for you now.
You could make the French translations for Visforms (which would be wonderful but is quite a bit of work, because there are many texts for backend and frondend to translate). I made a lenghty instruction on how to do so which you can find here:
www.vi-solutions.de/en/documentations/vi...slation-for-visforms
Alternatively:
For the frontend and only for the frontend, somebody already sent me French translations which you can find here:
www.vi-solutions.de/en/joomla-extension-...nente-visforms-3-0-0
Those translations are for Visforms 3.0.0 and miss four language tags, which were introduced with Visforms 3.1.0 but most of the texts are translated. The missing translations are minor changes for the texts in e-mails. You have to copy the fr-FR folder into the language folder (mentioned earlier).
Nevertheless I'm wonder, why you do not just get the englisch translation of COM_VISFORMS_FORM_SEND_SUCCESS? I have to explore that further.
Regards,
Aicha