Hello Christina! I hope I can answer you question to the best of my ability. As a speaker of 6 languages myself, I can completely understand the issues you face when trying to learn another one. Although there’s really no definite answer as to how one can maintain ones level in a language, the answer to me is really plain and simple. Firstly, our mind processes every language differently and as you said you start learning a new language off once you reach an intermediate level in another the issue that pops up is that before your mind is able to completely decipher and imprint one language properly, you move onto another. This results in both languages being a little here and there. The only way I would suggest you could maintain a reasonable level in multiple languages is by practicing it at every opportunity you get and try to select only about 2 languages at once. How well you’re able to speak a language will depend on many factors like your exposure to it, practice hours and practical usage. So I’d suggest not to overdo it and leave some room for your mind to process new vocabulary. I hope I was able to give you some clarity with my answer. Have a wonderful day!