No!

Okay, every now and then I see this question. Every time I get arguments for being arrogant when I give my answer. But, how many languages did you have to study in high school? I do not, know but here in your teenage years three languages are obligated school subjects. For everybody at the higher level. So everybody studies English, German and French at the same time, and does maths and geography and more. Now if you want to reach almost native speaker perfection you better would study only language, and once in a while you might confuse while and weil (because in German) and poison and poison (fish in French), but you will survive, trust me. Most people can study three or four languages at the same time.

I speak three languages and now am learning a fourth. All of my additional anguages I learned as an adult. In my experience, having already mastered a second language made it easier to learn another language in that language family (my native language is English). But ifyou haven't mastered the second anguage yet it might get mixed up in your mind with a third.

I first learned French and then Spanish. My French was solid enough that when I focused on Spanish that the French background helped rather than got in the way. Maybe that is partly becaue they share language roots. Now my Spanish is far more solid than French as I've been living in Mexico. However, when I return to work on French the Spanish only helps.

Now, beginning with Chinese Mandarin, I find it easier to learn the verbal part than any of the previous ones. I think that's because you brain has already formed additional anguage pathways...so it's actually less arduous in some ways as you add languages. Where I am having difficulty is with the writing...of course. That's because of the nature of the script is so different and much more extensive (and I am getting older!!! The memory becomes more of a challenge!).

I would say that if you are having two languages getting mixed up, maybe try to master one first and then turn to the other. 

It is of note that babies can learn any number of languages they are exposed to (listening and also having one to one interaction in conversation - yes baby sounds count as their part of the conversation!) THey will 'record' and integrate understanding and perfect pronounciation of each language without mixing them up. The brain is truly amazing! This begins to close up in stages...9 months, 2 yrs, 4 yrs then 8 yrs - as the hard wiring is established. 

 

The brain is the most amazing and mysterious biological machine ever created in all the universe.

The question is not about how many languages we can learn. Of course we could all learn multiple languages depending on how much time we have and of course the more languages we learn the slower we'll learn each one.

It sort of depends, it's probably a bad idea to start learning two languages from the same family. However, if they're not really related, you should be OK. Sometimes after I practise German and go straight to speaking in Portuguese, I get confused with word order and stuff (but never the other way round). It's a bit weird, but it doesn't last long and it's not really a major problem. And if you know a language well, learning another one from the same family will be made a little easier due to shared vocab, similarities in grammar etc.