I think Duo could be a good way to get started on language learning, but it is not effective on its own. What it lacks is an obvious way to graduate from its call and response mechanic to synthesis, as in creating your own sentences and participating in conversation.

Tandem was a good way for me to improve my Spanish to the point that I felt comfortable traveling. I dropped Duolingo pretty soon after starting on Tandem. Language learning is much more than memorizing words. Unfortunately, Tandem is also basically a dating site for many people, and scammers are using it as well, and this makes it hard to use consistently for language learning.

Once you get the minimal confidence that you think you could find your way back to the airport or bus station in another country, you really should just go visit. Couchsurfing really helped me meet people in many cities. I don't know if the community is still as strong, but it used to have regular meetups of people within a city who are interested in talking with foreigners. You don't need to stay on people's couches if you don't want to.

A lot of people seem to be learning English through multiplayer online gaming. I do not know if this approach works for learning other languages, as I am not inclined to participate.

I can't stress it enough, though. Any language learning approach that isn't writing or conversation is going to max out at a very low level.