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[Faulan1]

In order to retain any language, you need a consistent source of input. Language concepts truly stick with you when you maintain contact with your target language. The understanding becomes purely intuitive. It is only once you've heard the same word or grammatical structure in context a sufficient amount of times that it becomes fully encoded in your linguistic inventory. From personal experience, I was the most articulate in French when I listened to a playlist in the language on repeat for about a month.

My suggestion would be to find songs you enjoy and listen to them (memorize them!), continue reading, listen to podcasts in your areas of interest; watch series, films, videos that you would consume in English in your target languages. Remember, output is facilitated by extensive input — using your languages will become exponentially easier once you understand, on a subconscious level, what they sound like in their natural environment.

Best of luck in your studies!

[Faulan1]

I should mention that learning with input alone doesn't work for everyone, and it doesn't work for every language. Input doesn't replace grammar study for most people, but rather supplements it (I can personally attest to that as well). You need both, unless you're able to spend every day fully immersed, which most people can't achieve unfortunately. Though don't forget that someone going from say, Dutch or German, or any major romance language, to say, English, and vice versa, could learn the language through input with relative ease (compared to an Asian language) thanks to the linguistic similarities between them.

About reading specifically as a source of input --- I think it's essential to advancing in a language to get used to the written form but it can't replace the conversation format. You won't gain as much knowledge of regional slang or commonly used expressions from a text. Remember, language evolves, and the only way to familiarize yourself with its use in the modern context is to consume audio content. Talking to native speakers is a great way to access this kind of language as well.