Anki + Yomitan is the goat. Download some dictionaries of your target language, load them into Yomitan, and now you can search words on any website with selectable text and instantly make a flashcard out of the word. Watching a video in browser? Use asbplayer to load subtitles onto the video/movie and select the text from that too. Wanna watch not in browser? Use mpvacious with mpv.

Find yourself a card format you like and some cute addons (I use Ankimon [Pokemon]) for Anki so you don’t fall into that “it looks so dull!” trap because the SRS system it uses is no joke. I have years old vocab on there that I still remember, and medical students aren’t joking when they say it saved them.

I just love this app so much.