Indeed, this is a problem. Keyboard input in DarkPlaces (the engine Xonotic uses) is broken by design because it's american-centric, dealing with only ASCII characters and the usual special keys. See
this bug report.
There is a workaround but it's rather unstable. The thing with the keys you are trying to bind is that they are not ASCII characters, but they are still in the latin1 range. For some reason I forgot (perhaps because it's still single-byte encoding so the config can be read,) DarkPlaces still recognizes latin1 characters in bind commands when they are read from a config file. But there's a problem with this (that you might not notice), in that some latin1 characters lie in ranges of internal key codes that is also used by special keys. So, if you manage to bind a non-ASCII latin1 character's key, the bind might be shared with a special key such as page up/down, home/end, perhaps the F keys, and others (maybe even the keypad keys?)
Anyway, if you still want to give it a try, here's how you can bind those latin1 keys. First you need to create the autoexec.cfg file in your personal data directory if it does not exist yet. That directory is located somewhere in your personal documents directory and depends on your OS. The easiest way to figure it out is to open the Xonotic console (shift escape, works in the menu as well as ingame) and type
which config.cfg in there. This will locate the config.cfg file which lives in the personal data directory. So, next to that config.cfg file, create an autoexec.cfg file.
Then, you want to put your binds in there. For example,
Code:
bind "é" +jump
bind "à" "say I just pressed my a-grave key"
Now, this is important,
you need to save this file with a latin1 encoding. In some text editors you can pick the encoding in the "save as" dialog, in some others you have to set the encoding for the file somewhere in the menus. Use a better text editor if yours does not seem to let you pick the encoding. This matters, because utf-8 just won't work.
Don't hesitate to ask for help if you're having trouble with this. I'm also fluent in French if that can help.