Best & fast tech stack to learn for frontend

Hi im a backend developer that has almost no familiarity with the frontend languages or frameworks, i would like to start creating many projects this year, this means that i also have to learn frontend dev first, i was wondering if you guys can suggest me which is the best language (maybe js?) to learn and best frameworks to use that can save me time, cuz i need something efficent but easy to learn. My idea id to try to something similar to the 12 startup in 12 months, so having something that i can learn quickly and its fast to use it’s very important, i will use flask for the backend btw