(from a post at HTLAL, in which someone asked for advice on learning multiple languages over the summer)
Some people manage multiple languages at once, and some people get bored with doing the same language all the time, so they need a couple in order to switch between them to keep things interesting. But you should evaluate this for yourself.
Personally, my biggest problem in learning languages is keeping up the motivation after it gets hard and I stop seeing the quick progress I had at the beginning. If it takes longer, I won’t stay interested. For this reason, I like to concentrate on one language until I get to a certain comfortable level. For instance, in 2009 I concentrated on German for about 5 months solid, and was able to understand most of what I was reading, so then I decided to start Swedish.
I think if I had done both German and Swedish simultaneously, I would have spent a long time in that middle-zone where I can’t quite understand everything, and it seems frustrating. To keep my motivation up, I want to spend as little time as possible in that frustrating zone, so I try to do my most intense immersion then. I read every book I can find, and constantly listen to audio. I do nothing else, and purely concentrate on that language.
I don’t know what your current level is in any of these languages, but my recommendation would be to pick one first and power through it as hard as you can. Find every method possible to put your language superpowers to work. Start re-reading those posts where some crazy person says “ya, I learned 5000 new words in a month” and be inspired. Once you can sit down with a new novel and enjoy it with ease, then you will have gone through most of the hard stages of a language, and you’ll know what to expect for the others.
Once you have a satisfactory experience of being able to enjoy a novel in a new language, or something else that might signify “victory” for you, then perhaps at that point it would be a better time to try to take on multiple simultaneous languages. When you’re working on those languages, you can look back on your previous victory and use it as motivation, and as proof that if you just keep working then you’ll get there in the end.
I commonly feel a strong desire to work on many languages, mainly because I’m eager to be able to speak those languages to people around me. Language-learning (and especially polyglottery) is a long-term project, and you have to treat it as such. If you want to learn 5 languages, budget 5 years. If, after 5 years, you’ve learned a decent amount of those 5 languages, you’re actually huuuugely ahead of most people, and you will have accomplished it very fast. Personally, I just really want to be there at the end, speaking with many people in their own languages, but I know that it’ll take a little while.
It’ll probably be more satisfying to speak 1 language after 1 year, 2 languages after 2 years, 3 languages after 3 years, etc…rather than 0 after 1 year, 0 after 2 years, 0 after 3 years, until eventually you’re able to speak all 5 that you were simultaneously working on. This is my experience, anyway, since I spent multiple years with 0 functional languages, despite dabbling in over 10 of them.
Getting awesome at one language first will also help your language-learning skills, enabling you to learn the rest faster. Spending multiple years being a beginner at 10 languages only helped me get good at blasting through the beginner portion of language learning, but then I’d just drown in the intermediate “frustrating” section until I’d quit and start something else.