I'm specifically looking for something that is both easily customizable and accommodates a bunch of multilingual glossaries.
You can check out the Duolingo or Dictionary if you're on Android. The Dictionary app includes all kinds of additional information including translations and word origin. Duolingo is a complete language tutor app that works really well. It will start you at a higher level if you're already a little fluent in a language and has a ton of great exercises to help you learn the languages. If your on a computer you can download Google Chrome and get the same two apps and run them without having to start the browser every time. Neither app has offline multilingual glossaries, but they're both great resources for learning other languages for free. The other two programs you may want to check out, if you're on a PC, are called Lingoes and Ultimate Dictionary. Both free programs offer multilingual support, Lingoes looks like the better fit for you but I thought I would mention both. Can't link to the last two apps (do to StackExchange restrictions) so you'll have to do a Google search for them. Hope this helps!