I guess I learned Swedish the old-fashioned way: memorization, studying grammar books and talking to the people. That was over 50 years ago. Today, I am translating a book from Swedish to English. And I can read every word on that IKEA sign. I live in the states. I haven’t been back to Sweden, but I have kept up with the language via the Internet.
I have a method that works similar to yours, but doesn’t involve any more technology than a paper and pad. (And I am a tech guy.) When you are out and about town (You have to be in the country where the language is spoken.) I write down every word a see or hear that I don’t understand. This is easy because the country is geared to walking and public transportation.
This is a way to find the most common words, based on natural selection. But, you are right; if you learn the most common words, it will take you far. I got my list from real life. Kids and newspapers are a great source. Language and grammar books have already made this selection for you. A first year high school language course has already come up with the 1000–1200 most common words and will teach them to you.