You never fully "forget" a language if you don't use it for a while. I have used many languages over my career. I did Java programming for five years. I switched jobs, and thus languages four years ago, and now I'm doing mostly PHP. I could still do Java if I had to. Here's the thing you'll come to realize: just like spoken languages, programming languages are mostly similar. There are nuances to each one. But, if you learn one or two really well, you'll be able to pick up on others fairly quickly. For example, if you know Java, C# should be pretty easy to learn.
This is the one point I always hated about job descriptions and clueless recruiters. They say, "must have 5 years experience with X language". What if I don't? But I have something better: 18 years experience with many other languages. Unless X language is so vastly different, there should be no reason I couldn't pick it up in a few weeks' time.
Learn concepts. Understand there ARE differences, but also understand there are also MANY similarities that cross language barriers.
Personally, I think it's great you want to be full-stack. I wish more developers were. From a management perspective, it's very frustrating to not be able to assign projects to certain developers because they don't know the X layer (e.g. database, front-end, etc.) I'd so much rather have a developer who is able to do everything, than one that specializes in a particular area. But that's just me. Not everyone shares this opinion.