” If I update le Quix extension, will my settings disappear?”
No 🙂
“If I add the French language to the administration part of the joomla site, will that be in conflict with the en-GB settings made in Quix, for example?”
Do you mean using the site as multi language site? If yes, instead override the language via code you can choose the solution that using joomla language override for special language 🙂
” In your EDU template, the button “Subscribe” is in English and I’d like to change it into French. Where can I find the configuration file?”
You can use joomla language override with language constant: QX_CF_SEND
Example this https://docs.joomla.org/J3.x:Language_Overrides_in_Joomla you can choose FRENCH language when overriding instead English
