To maintain languages you have learnt, it helps it you use them as often as you can. This might involve reading novels, listening to the radio, watching films, writing a diary or blog, and/or conversing with native speakers or other fluent learners. The less the use your languages, the more you will forget them, or at least you will find it increasingly difficult to remember vocabulary and your fluency might suffer.
Fortunately you’re less likely to forget languages in which you are very fluent, but there might be some interference from other languages you’ve been using more frequently. For example, I find that Welsh, the language I hear and use the most at the moment, tends to impose itself on my other foreign languages.
According to recent research, a language you believe that you’ve forgotten is in fact still in your memory. Your ability to retrieve the memories deteriorates over time due to lack of use, but they are still there. If you start using or studying the language again some aspects of it are likely to come back to you, and other aspects will feel familiar, which will make them easier to learn.
I studied French at school for example, but have used it very little during the past 20 years. Recently I joined a French conversation group and have found that my French is starting to come back, even though I’m not as fluent as I used to be and some vocabulary has become difficult to recall. Many other people have had similar experiences and have found that languages they believed lost have come back, at least to some extent, when they find themselves in situations where they have to use them.
So even if you are not able to maintain your knowledge of your foreign language(s), you can revive them or will find it relatively easy to relearn them if you need them again.