The keyboard shortcut for inserting comments (Command+Option+C) does not work. This is independent of documents (does not work for any document). This is the official keyboard shortcut (i.e. the one mentioned in the "Insert" drop-down menu.
No repro with Version: 5.3.6.1 Build ID: 686f202eff87ef707079aeb7f485847613344eb7 Threads CPU : 4; Version de l'OS :Mac OS X 10.13; UI Render : par défaut; Moteur de mise en page : nouveau; Locale : fr-FR (fr_FR.UTF-8); Calc: group Cmd-Option-C in Writer inserts a comment for me. Also works in Calc and Impress. You could try re-setting your user profile if you have upgraded from a previous version of LibreOffice, or else try starting LO in SafeMode to see whether the problem is linked to your LO user configuration profile. In all of this, I am assuming that the keyboard combo you mention hasn't been assigned to some other shortcut (system shortcut, or other function, e.g. assistive technology, etc)
@Manuel : might be useful if you also let us know which version of OSX you are running.
@Manuel: oh, and additionally, whether you have installed any langpacks (and which ones if so).
@Alex Thank you for the quick reply. I have looked a bit more into it today and it turns out one of the other applications I am using (Spectacle) had registered another action (center window) over the same keyboard combination. I couldn't tell that earlier because I always use LibreOffice in fullscreen. I am sorry for wasting your time. This issue should be closed.