As any language, the core is "why" do you want to learn it. Is it to add it to a list and that's it? Then you might struggle by creating todo lists or play pretend on Duolingo.

On the other hand, if you do have a goal in mind try to do tiny bits of that.

My goal for natural languages is always connecting with another culture at a deeper level than just using English. If that's the case, you get someone to talk/write to and slowly do it. It won't be instantaneous or dopamine fueled but after a few years you might realise that you've been chatting with someone completely in their language without major hiccups.

For programming languages, I understand that filling a CV is tantalising and useful, so you've got to come up with projects and things you'd actually like to be doing with such a language.

You could say you want to pick up COBOL for a future job, well figure out what would make sense to use it for and go with that.

And if you really cannot think of anything, then you can fall back to make something up: make a game with such a language (even better if it is not meant for games), automate something, recreate a small tool which you find frustrating. And even if after you have read this and still cannot find a thing which gets you, maybe learning this language is not within your current interests and you might start considering to move on.