I have a RoboHelp project (RH 18.104.22.168) created by importing an HTML Help project that I use to generate WebHelp. The HTML files, the .hhc, .hhk, .hhp files have been translated to French (from English). I have changed the language in the project settings to French (France) and I generate the WebHelp output. All appears to process correctly except the TOC does not expand and collapse correctly. Some first level entries expand with one click (as expceted), some entries only expand after other entries have been expanded, and some entries will not expand at all, and once expanded, some entries will not collapse.
I have used the same process to generate other language versions and have not had the same issue. I have been searching the forum to see if similiar issues have been reported but cannot find anthing similar.
I had something a bit similar with RH 7, except it wasn't limited to one language but Firefox browsers. That problem was solved by installing the latest RH patches. You have a later version of RH and the problem is only in French, so the problem is most likely in the French help project.
What I'd do to troubleshoot it: Make spare copies of the help projects, just in case you make it worse and not better.
It can happen that a translation agency does something to the translated files, that does not work in your system. It can be very obvious in a side-by-side comparison, in a comparison tool or just Notepad, or it might be a bit more hidden like wrong character encoding.