Recently, I’ve been deciding what language I’ll study next, since I don’t want to limit the roster to just English and Japanese. What point/level should I get to before making that kind of change, so that I’d be able to maintain that current level while advancing my proficiency in the other language?
I don’t think it matters. The main thing is that you have to trick your brain to move beyond bilingual mode. The brain is lazy and wants to split all the world’s languages into two: your native language and everything else.
When as a child I went to Norwegian camp, I forgot all my pre-school Spanish. The next summer, I went to Russian camp and forgot all my Norwegian. Then I spent eight years studying German in school for an hour every day. I thought I was fluent. I thought I would never forget.
But the next year I spent abroad in Denmark. German was a great help in learning Danish, far more than my forgotten Norwegian, but my learning process was largely a matter of replacing German vocabulary with Danish. That was when I realized how my brain had fooled me.
For me, Japanese is a “final language.” There won’t be any next language. But that hasn’t stopped me from dipping into French or Portuguese on the side, or rereading some German poems or Danish fairy tales. But now I am more careful about compartmentalizing.
I think the best time to start a new language is when your current one feels solid enough to keep going without daily studying. Like, if you can watch shows, have basic convos, and read most stuff without too much effort, you’re probably good to go.
But if you still forget a lot after short breaks or need to constantly look things up, it might be better to wait a bit. You want your base to be strong so it doesn’t fall apart when you shift focus.
Basically — get to the point where it feels natural, not forced. Then go for it! がんばって。
I'd say that it depends on your time and resources, otherwise there's no reason not to learn a new language if you feel like it. It's more of an issue when you want to learn a "similar" or related language to the one you are already studying without having a solid base (like if you started learning Chinese, since you'd end up mixing both up quite a lot).
I used to learn four languages at the same time which were japanese, French, spanish, and german. I was at a snail's speed, after which I realized that I should only concentrate on 1 language.
