I used to think about C # / Python. But then I learned that the automatic memory for the open-world game will stop.
BGT will be able to cope with a huge map, and work at the same time in 2000 objects?

I think the first question to ask here is, how much programming experience do you already have to start with?

As for Python, it handles memory management and free's up memory using a process called "Garbage Collection". If written properly it will never "run out" of memory, though that depends on the system your working with and how you write the code. Contrary to what some people may think, open world games are not loaded into memory all at once because this would require a vast amount of memory and processing power that target systems usually don't have. Typically such games load only the players immediate area into memory, removing and loading nearby sectors as they move so the world actually loads around them, things like characters or game persistent variables are then handled abstractly in the background. This is partially why games have "draw distances", which sets how much of the world to load around them.

Jonikster, visualstudio is correct that the short answer is yes, you can use BGT to make those types of games.

If you were going to type out a paper for school, you might ask everyone if you should use notepad, wordpad, or microsoft word.  Really it doesn't matter which typing program you use because the paper all depends on what you have in your head that you will be typing.  In the end the paper that you turn in will not be different because you typed it with a different program.  On something like notepad you can just immediately start typing because it is less complicated of a program.  With MS word you may need to set up your page header, formatting, font size, and a few other things in order to make it look like how you want in the end.  So it really depends on how much extra "stuff" you care about doing before you type your paper.  If you want to spend the extra time to set up MS word, then in the end you can do a few extra things with how your paper will appear.  If you use notepad you can start typing right away and get done sooner, because you didn't have to play around with all those extra options.

It isn't the best comparison, but choosing between BGT and C++ could be thought of in a similar way.  If you use BGT you can probably start making small games sooner, because it is meant to do many things for you.  You could think of this like notepad where you can get moving much quicker, but miss out on some features that you may or may not even care about.  With C++ you are probably going to spent much longer just learning the basics, so you won't be producing any games anytime real soon.  If you do take the time to learn it though, you will be able to do some things that were not built into BGT.  As stated above, BGT is meant only for audio games but C++ can make anything.

Now I will add a bit of advice that is only my personal opinion.  I often hear people say that people should learn C++ instead of BGT because C++ code is something they can use later if they have a job as a programmer.  Basically the idea is that they don't want you to have to learn BGT and then learn something else later.  I would personally not worry about needing to learn something extra, in fact, the best programmers I have ever known knew a lot of different languages/approaches.  Each new one you learn helps you think a little differently about coding strategies.
I mean no one ever says "Don't play Monopoly because the controls are different than FPS games you might play later."  Learn the controls for Monopoly now and then learn the controls for the next game when you come to it.  It's a normal part of life, and it seems strange to me that so many people try to avoid that when it comes to programming languages.  Okay, that's the end of my little opinion on the matter.