I think some folks wrongly assume there's one tool that will make them proficient in a language (eg Duolingo, or a language course), but I think there isn't. Just curiosity.
E.g: I started learning German via Nico's Weg on DW. Was fun, but I started wondering if it was enough. Then I stumbled on threads like this and realised my mistake. Since then, I've used DW, Duolingo, Learn German with Anja (YouTube channel), and many other random blogs and videos.
This is if you're looking for proficiency, btw (the definition of which also varies by person). If you just need to know some basics, or you're learning for fun, one or two tools might suffice.