How many do you speak? Learned any as an adult? If yes, any tips and tricks to accelerate learning (other than moving countries)?
Take what I say with a grain of salt as I'm not fluent in anything but English yet. But for me the "tips and tricks" are largely counterproductive. What you want to do is to understand how the language works on a fundamental level. This isn't as hard as it sounds. You just need to understand why the basic parts of a language work and interact the way they do. Everything gets easier after that, and largely comes down to memorization of vocabulary and exceptions (use your favourite tricks here to help you), and the #1 most important thing, practicing with another speaker of the language. The more you actually use a language as a tool to communicate, the better and quicker your brain gets at using that tool without "thinking" about what its doing.
Apps don't work, other than as a piece of a larger learning strategy, because they can't replicate speaking with another person. They are largely just time-sinks designed to keep your attention on the app to make money for the creators.
Learned all in the past few years.
Go for Pimsleur to start, plus a little bit of Duolingo. Then start going to language meetups (this is the most important after you're approaching B1), reading up about grammar a bit, using Italki for online practice, and think about traveling to the country.
You want a balance of natural speaking ability, but also want to understand the general gist and way the language works so you can reason things easily if needed to - to fill in any gaps.