Apps like duolingo will only take you so far. Youtube channels will only teach you certain things.
If you cant go to the country where the language is prominent, then you're gonna have to do some boring shadow writing and speaking. It takes a ton of repetition, making mistakes, and listening to how it is properly done.
But you will be missing out on a key factor: cultural context. Ever meet a weeb who speaks really weird Japanese? Its because he learned from a weirdo youtube channel or anime. He doesn't understand nuance, cultural norms, references, idioms, or historical things that go into language.
So my best suggestion, watch people talking, write what they say, repeat it. Every day. Its boring, but unless you actually know multiple native speakers, it is all you have.
Im a second language teacher, btw.
Youre not wrong.
I realise my reply may have come off as flippant, but I was being genuine when I said that moving to the relevant country would really help.
Ive been in Vietnam less than a year and while learning the language sometimes feels like a very slow process, Ive learned enough to pass the driving test over here, which is only available (theory and practical) in Vietnamese, so Im definitely getting better.