Can I learn similar languages at the same times?
But when we were at school we've already done this. English and french and spanish and german and latin languages . those have similitudes, but at that time did you ever asked this question to your professor ? I don't think
From my personnal experience, I'm french, I've learned english at school and spanish. I losed most of spanish because didn't maintain it, for english I'm more fluent in writing because I keep it until today. I can't speak fluently because I don't use english in my daily or casual life. I've started to learn korean 7 years ago and keep it regulary, I'm not fluent but I can keep a conversation in korean with my friend, understand and I use it to maintain and learn japanese. I find speaking japanese easy, not because it is, just because I learned a lot through all those times, your brain's more flexible, you know more how you can learn more effectively
What You've acquired its forever, it'll jsut come back if you need it.
My compagnon acquired spanish in 6 months when he was young because he goes to spain for vacancy and his father put him with other spanish children, he didn't know anything, but he can understand and speak spanish even if he don't use it and don't try to maintain it, just using it one or 2 times with his father a year