When do you not have the feeling of wasting your time?
Giving you a fair answer, I don't feel like I'm wasting my time when I'm doing something with an objective or strong ambition. Once I've achieved my goal, I get the feeling that I didn't waste my time in vain.
I'll begin using your own example: learning a new language. I don't speak English perfectly – not even close – but I do feel like I haven't wasted my time learning it, which took me more than ten years. Another one would be reading a good book. A good book that brings you some value.
And now, my opinion about it is that
it's easy to feel you're wasting your time in whatever you do. If you see it through a contemporary perception –
strongly based on my personal perception. There's a lot of 'paths' out there. There's no right or wrong, and the worst of the evils, there are too many distractions. There's always something new, or better.
You can be so many things and at the same time, you cannot. You can try 450382 things, and there will be something else that's going to make you ask this same question over and over. Am I wasting my time?
What's worth or not, is subjective. In sum, if you're doing something that adds value to yourself, then you shouldn't feel like you're wasting your time. Oh, and by 'value' I also mean it subjectively. Does it bring you knowledge? Does it entertain you? Does it make you feel happy or perhaps accomplished? And so on.