FYI, that character encoding is UTF-8; spelling it correctly should help you when you do searches on the web. (And it's explained on the unicode.org web site, if/when you want to get into the details.)
It seems that you should start by examining the CSV files that they send. You can browse the content of those files using a hex viewer (or hex editor, but you should not change anything). Hex viewers/editors can be downloaded for free, if you trust them not to contain malware. I found a web site that offers this capability online.
I don't know French, but I created a file containing French accent marks (thanks to the webpage How to Type French Accent Marks: 42 Keyboard Shortcuts) as UTF-8 and then used the hex editor page to view it and capture this partial screenshot:
Like most hex viewers, it shows the data on the left (one-byte storage chunks shown as two characters each) and its attempt at a visualization to the right. Notice that I highlighted the first byte of a c with cedilla in français. (If that byte had been hexadecimal 63 instead of the C3 shown, it would have represented a "c", and would have been followed immediately by a 61 representing the "a".)
Talk with someone at your company who may have a hex viewer/editor. If you have concerns about uploading one of your CSV files with possibly confidential or business proprietary information to such a web site, don't do it.