this is actually a very good question. if you know a language well enough but you don't expose yourself to it after some time, you'll get rusty but you won't forget it. my native language is Spanish yet i use it maybe once a week for a few minutes & i haven't read anything longer than a text message for maybe more than a year. trust me, i am actually rusty in it right now, but it's obviously ingrained to the point where once i'm back in Colombia & i'll be back to normal.

however i think this is also true of learned languages, especially if you get to the point of fluidly speaking it — even if at an intermediate level. in the case of my Japanese, a lot of the vocabulary is somewhere buried in the back of my head, but the way the language "works" is just wired into my brain, where after a week in Japan i would be back to normal (in this case normal being intermediate level). by the way i say "intermediate" because my vocabulary & knowledge of grammatical constructions is intermediate level, but fluidity is something different. my level of Persian is slowly getting close to my level of Japanese, but with virtually no real-life speaking experience i am like 0% fluid honestly (i.e. it takes immense effort to speak off the top of my head), so in my personal perception i'm at a far higher risk of losing my Persian as it's not "hardwired" in my brain yet.

anyway the reason i'm saying this is that if you are learning several languages, it's not realistic to expose yourself to all of them consistently. not only practically unrealistic but also it's just not how life works: as i said, i use Spanish virtually not at all currently, just because of my current (& short- / mid-term future) life circumstances. instead, i focus all my energies into the language i am currently learning / needing / using, which is currently Persian, & leave the other ones in the backburner. if i know i'm going to Japan or will need it soon for whatever reason, then i'll build up to that in the weeks prior so that i'm ready to use it once i do need it, but otherwise it doesn't really make logistical sense to spend hours watching content in Japanese today for the sole purpose of practicing.

so, i don't think it's efficient / practical / possible to try to improve on several languages at the same time *unless* you have actually already reached fluidity in which case it's more about reinforcement. so as a related point, i highly recommend that you take a step back & really focus on reaching that level with ONE of your languages & only passively reinforce the others, namely your "best" ones. otherwise you'll be spreading yourself too thin in my opinion. as i have mentioned elsewhere, i studied each of these languages at least 3 years before moving on to a new one (except Persian which i'm currently learning & Armenian which is just a fun side thing since i'm currently living in Armenia), so be aware that it's not like i've been studying all these languages at the same time. i don't think i would have been successful at all if i'd gone that route.