What can I do with my lingots?
I have 27 lingots earned in the first 4 days, and I already bought the timed practice and the double or nothing.
Can someone tell what else can I do with my 27 lingots?
Frankly, there's not that much to do with lingots.
You can buy streak freezes, which let you keep your streak going for one day, even if you don't meet your XP goal (you can only have one day of streak freezing enabled at a time, though). You can buy timed practice, which is really good for forcing you to recall words and translate between languages quickly, once you're feeling okay with the content of the underlying lessons. You can buy "bonus lessons," though the availability of these depends on which course you're doing, and I don't think any have been released for Japanese yet (which isn't surprising, since it's still in beta). People sometimes give them to each other as a thank-you gesture for answering questions or otherwise posting helpful comments in the forums and sentence discussions. If you're on the app, you can buy a couple of different outfits for your Duo owl to (sometimes) wear. You can gamble with them by betting Duo "Double or Nothing" that you'll keep your streak going for seven days, although if you win the bet, you're just left with more lingots.
That's about it! I think I've read in various interviews that Luis the rest of the Duolingo staff are aware that the lingot economy doesn't entirely work, and it's on the list of things about the site that they'd like to improve at some point. I'm glad that they're aware of the issue, but I'm also glad that they've been making it a lower priority than, for example, releasing and improving the Japanese course itself!
So far, I've used them only for streak freezes and double or nothing, and I've given a few away. But once I have progressed enough in my languages, I plan to use them for extra lessons and timed practice, and I expect that I'll be able to use quite a few of them for those things... For right now, my focus is on getting through the courses and then practicing them at regular speed. But once I have that covered, I'll go for getting faster and learning more.