How to learn a new language quickly?

Surest way is to speak it with someone who already knows the language

Practice makes perfect. I've tried a few apps over the years but what really made a difference for me was signing up for a language school that doesn't just focus on the written part, but really puts the language to practice as well during discussions etc. Another major help was being able to practice with native or bilingual friends after school because that's what you learn the most from.

If you want to learn french. Take a job in a french company for 1 or 2 years in France, Belgium or Swiss. You'll become fully fluent!

There are few apps online which can help you with learning new languages and along with that you need to practice it with someone who know that language. You can find many communities on social media

I've found duolingo to be quite helpful for languages that it supports.

I think the best hack to incorporate is consuming media natively produced in the language you're trying to learn.

focus practice assignment and focus

You don't get to learn a language quickly. It takes hundreds of hours to become fluent. But you can avoid wasting your time and focus on methods that work. It'll make the difference between starting and giving up because it's too hard versus seeing results, maintaining your motivation and getting to fluency after hours of rewarding work. The biggest mistake I see people make is learning too much from text and studying too much, waiting for them to be 'perfect' to finally speak. What seems to work really well is to learn sentences you know you will need, go use them even if you have less than the vocab of a 2-year old, and use opportunities to speak. Learning languages is a beautiful way to travel (mentally and/or physically) and experience a whole aspect of life that's closed to monolinguals. I would compare it to knowing love or playing music. It's a beautiful part of life.