Learning more than a language at a time
Is it fine to learn more than a language at a time? To be more specific, my case is learning French at level 5, and interested to learn German from scratch. Thanks in advance.
What I would do (Or more specifically, what I'm planning to do) is to learn one language, master it (finish the language tree and get 80% or higher on the Duoling certification), and then move on to a new language, while still working on the first one with the practicing weakest words in the vocabulary section every day. I don't trust myself to be full out learning multiple languages at once without mixing them up and getting rules confused...
Personally, I think it is best to stick to one language at a time. Most of us have limited time and lack the ability to spend time learning two languages simultaneously. Moreover, languages are hard to learn. Thus, learning two languages at one time will be challenging.
Also bear in mind that languages take time to master. Completing the Duolingo tree won't make you proficient in a given language. Repeated practice over months (or maybe even years for non-hard core learners) is necessary to become proficient in most languages.
With that said, we all have different goals. If you are in no rush to learn a language and are just learning for fun, you might be able to get away with learning more than one language at a time.
You'll have to try and see if it works for you.
I've heard some people find it impossible to learn Spanish and Italian at the same time because they're so similar. I don't have difficulties with that.
In the case of French and German, they're from totally different language families so you won't have problems with learning false friends, at least.
After early dabbling, I decided to go to 10 in each language. If they add more languages in time, I will go to 10 in those too, but otherwise I'll start going to 15 in the ones I've done. I find that I benefit from each new language. But I don't regularly switch from one to another because I don't like to fail lessons (all that effort for no XP!).
At first I was worried that my old languages would degrade and it would be like starting over. But I've done some experimentation and it's not really that much effort to catch up on your topics once you've done them initially. They just provide a great way to review/refresh what you know.