I wonder if you will have learned multiple languages in the end, the order of learning each of them has any influence and if it does, what is the most effective order to learn.
For example, suppose that you are a native Japanese speaker with decent English proficiency (B2), and are going to learn the following:
It is reasonable to conclude that the order of languages closer to his/her native language is:
Korean < French < Russian < Arabic
French < Korean < Russian < Arabic (since already fluent in English, which is closer to English)
So I want to know whether there is any research that the order of acquiring languages has any influence on the effectiveness of acquiring all of them. I mean, is it more effective to first learn a language closer to your L1 and then gradually learn more difficult ones, or learn difficult languages first and then go with easier ones, or anything others that are considered more effective (such as learning a language that is closer to the one you just learned next, or deciding by the closeness of pronunciation or vocabulary instead of the total amount of time to reach a specific goal)?
In effective, I mean the total amount of time required to reach B2 or C1 (in all of them). And in closer language, I mean the expected amount of time required to reach the specific level, such as the relative difficulty chart on FSI (for English speakers).
The reason I think learning difficult ones first is more efficient is because if you learn an easier one first, you might forget it while learning a difficult one, which takes more time. For example, suppose that it takes 6 months to acquire Korean but 24 months to acquire Arabic. In this case, learning Korean first would give you 24 months of a pause after that, but learning Arabic first only gives you 6 months of the pause.
In all the languages, I'm well motivated and want to learn all of them eventually. I have enough time to devote and can move to the country to join an immersion program.