The simplest answer is, you don’t (add c++ or php or … to HTML, or run any of it in Notepad++). It appears you have a fundamental misunderstanding of how a page gets from a webserver to your web browser. This isn’t the forum for the details, but here are some hints. (It’s not rigorous… just some thoughts to get your mind moving in the right direction.)
Notepad++ is an editor, which you can use to edit the source code (for your HTML, CSS, JS, C++, PHP, Python, Ruby, Perl, etc). Once you have the source code, you can often debug them locally (assuming you have the appropriate tools) – but the end result is usually viewed through a local browser looking at your local copies or thru a local browser requesting the documents from a development server; Notepad++ can be given hooks to do portions of the testing (it can send source code to your c++ compiler; using the Preview HTML plugin, it can render into your Notepad++ window, so you can see gross functionality of the HTML; it can open the current HTML in your favorite or least favorite web browser using Notepad++'s Run > Launch in Firefox and similar, etc) – but Notepad++ doesn’t “run” the HTML+CSS+JS: it launches the helpers to do their thing.
Now for some more clearing up, regarding your questions about the other programming languages you mentioned: browsers (primarily) understand HTML, CSS, and JS. The server understands C++ (after it’s compiled), PHP, ASP, Python, Ruby, or Perl – those server-side technologies are used to generate HTML, CSS, and JS (or you can just have static HTML, CSS, and JS as files on the server). The server side code (C++, PHP, …) can be programmed to access a database using SQL or similar query languages, and manipulate the server-generated (non-static) HTML, CSS, and JS. (I think I’ve even heard that you can directly query a remote database using JS from the browser, but I don’t have much experience with that.)
The server side code will generate HTML/CSS/JS, often using inputs from forms submitted from the browser back to the server, and serves this HTML/CSS/JS to the browser. The browser interprets the HTML and CSS (and XML, XSS, …) to format the information from the server, and can run the JS locally (which can manipulate the HTML and CSS inside the browser and thus change what’s there locally).
To test a fully featured website, with server-side code, databases, and HTML/CSS/JS output, you will need basically another server: all the same tools with the same configuration on a local machine, or a second “development” server remotely accessed, or a private/hidden development area on the existing server to make sure you don’t mess up the released website. Once it’s all fully tested, then you release it to your production website, and away it goes.
There’s a lot more to all of this, none of which is on topic for a Notepad++ forum. If you need more, find some good books or classes on web development.