Hmm.
I learned Hebrew in Duolingo and I've in fact spoken many sentences to natives. All the Israeli women we know are always negatively comparing their American Jewish husbands to me, the goy who learned Hebrew from Duolingo.
Granted, those natives are either married to me or related to someone who is. So maybe that component is essential.
Ok, it may work if you're in the language ambient, IDK. The situation of most learners is different: we live in our countries, there are very few if any speakers of target language, or they're too busy.