There are a lot of really great options but most of the good ones aren't free. My first recommendation is to use an opensource Wiki app. They allow you to document the software and add to the docs as support questions come in.They are designed to be searchable, your company can host one instance with multiple products,and generally, most wikis provide an ...
He asked for Open-Source Project, so Visual Studio should not feat.
Would recomend Code::Blocks, it´s free crosplatform IDE and has, as far I know suport for CMake and all the other features. No Idea about Clang, but think it should also be suported.
I recommend Visual Studio 2022, I have used it personally. In my opinion, it is the best IDE for your requirements.
Visual Studio comes with its own built-in autocomplete feature - IntelliCode. You can also use plugins like Tabnine for auto-complete.
Visual Studio also has Interactive Debugging, Integrated Tools, Syntax Highlighting, and good file management....
I'm using Getscreen.me for this purpose. Just select the desired language on the panel, put the cursor on and the service shows online translation of the selected text. Recently I connected remotely to a client from China - the function was very useful. A pretty good alternative to Teamviewer or Anydesk, but cheaper.