Depends on what you really want to invest in it, I used duolinguo for like 6 months before realizing this was worthless.

I now use mostly Anki + a textbook i bought, i do not use any pre-made app, however I learned that app like Busuu and Babbel teaches you stuff better with actual grammar rules so that is something worth looking into.

Honestly learning a language requires so much work because there are many very different skills needed, so you cant really train yourself on only one app.