This is not to brag, it’s just to show you it’s not impossible to know many different languages.
I teach four languages a week. English, French, German and Dutch. In many cases I have to explain things in Slovak during my lessons. Some of my students are Czech. I understand Czech about as well as I understand Slovak, though I do not speak Czech (I would if I were to spend a few months in Prague for example).
I regularly check the news in Russian, Spanish and Slovenian.
I can’t speak Spanish, but I have no trouble reading it.
The same is true for Italian, but for some reason I am not drawn to Italian, so I do not actively read Italian.
What helps with Roman languages is the fact that I had an obsession with Latin when I was a teenager.
I actively use 5 languages almost on a daily basis.
What I do to maintain my fluency is reading the same books in many different languages.
In the picture you see a copy of one of my favorite books: Bright lights, big city. It’s the French version. I have read it in German and English as well. If I can find I will read it in Czech or Spanish or Russian or Slovenian.
I don’t read books in Dutch, well, not often anyway, but I do read some articles in Dutch for my students.
I can read Serbo-Croatian and have been able to have conversations with people about the war in Yugoslavia in Bosnian.
I can sort of read Danish and Swedish newspapers, though it takes a lot of focus.
I would like to warn people that when people claim they are completely fluent in five or more languages they are probably bullshitting you.
I would say my life has always been about languages.
I remember being six years old and being magnetically drawn to tourists and trying to figure out what they were saying.
So I never believe people who claim they are totally fluent in many (more than 5) languages. It’s possible, but it takes a lot of effort to maintain fluency.
For example I used to be fluent in Slovenian, but I haven’t spoken the language in ten years. So naturally I lost my fluency.
Given that my life and my job are all about languages I would say it’s nearly impossible to maintain complete fluency in ten languages (this used to be my goal).
There are things you can do, if you have a lot of motivation and time, but it will be a huge challenge.
I have tried gaining fluency in other languages than the ones mentioned above, but – in part because I do have other hobbies than learning languages – I have given up on that.
Bottom line: yes, you can become a polyglot, but let nobody fool you into thinking it’s going to be easy.