Well, go to global config. Enable debug language and then look at the output in the frontend. Do you have any errors in the language files?

I think the problem is with the file JROOT\language\en-GB\en-GB.lib_compojoom.ini.
It is loaded with the UK English version, but not with other languages. It does not exist in a translated version, which means that usually the english version is loaded, but that does not seem to happen.
See dump of loaded ini files on the next page.
There are no translation errors

lib_compojoom shouldn't be the problem as the ccomment strings seem to be loaded. Are there any langauge errors output in the debugger?

No. There are no translation errors or missing string messages. I spent a few hours trying to find anything, and the only thing I could find is that the main comments form is simply not loaded at all (I compared the html output of the english version with the dutch/french/german output.)

if you look at the nl version of com_comment.ini - is it in dutch?

The language files contents are in the correct language.

I'm out of ideas. I'll need to debug on your site. For this please provide me with access credentials.

Regards,
Daniel

That is not possible because it is on my local laptop.
I have already solved it by using a forum solution instead, as it is only meant for a very temporary project (yes, caused by the Corona virus), and not for a production type environment.
I am quite sure that it is a problem caused by a specific version combination of PHP, Joomla and ccomment, as I have not had this problem before.

Thanks for assisting me. Really appreciated!

All right. Thanks for letting me know!

Just wanted to confirm that it was indeed a combination of server side versions of PHP etc.
Just did a new install on a different server and had no problems at all. PHP 7.3.16, joomla 3.9.18 works fine.