What to do with all the lingots :( There are only 3 items in the shop.
Unlikely, given that none of the Incubator-made courses have bonus lessons. Only Spanish, French, German, Italian, and Portuguese have them, and there seems to be no sign of expanding the feature to other languages.
That said, I'd happily fork over all of my lingots to get more kanji in the Japanese tree!
I don't think that's a very good idea. Since Japanese is a very different language, if you take a break for a while you might end up forgetting a lot. If Japanese is a language you really want to learn and have a passion for, stick with it. After all, Duolingo is just one resource out of many. If you want to learn more, I highly recommend the Genki textbook series. It's by far the best Japanese textbook I've ever used.
Hope this helps! Good luck! 頑張ってください！
What others said. Save them up. :) You can use them to give to others- especially for those tutors out there who explain something helpful (like particles- ai!!!), or just made your day. :)
For yourself, you can use it to save up on a day that you can't make it to your device (pay for a streak freeze).
You can use them towards future courses (necessary if you answer so many questions wrong in a row in other courses- like Spanish and Italian).... they are used like coins to continue playing a game in an arcade.
Don't stop with Japanese! Keep going at it every day. I totally agree about the comment about Genki Book. I am JUST starting Japanese, and it is the book I am leaning upon the most right now. I also just signed up for a tutoring one-one-one via iTalkie every other week, and just signed up for Busu. Therefore, I had to cut my "coach" down a bit. I was going 50 XP a day, but now I am down to only 20XP. Duolingo is definitely helping, but I think I need to put more effort and time through other means. Still continuing Duolingo, though。
Find out what works best for you, but keep moving on.
Just a tip... maybe if you have a lot of lingots... you can use the lingots to do a Streak freeze so that you can maintain a streak... and also earn more lingots so that you can just use it if you need help! Or if you want to... you can be nice and just give some away. (And I don't need the lingots! I'll work for them!) By the way... I see that you have Japanese 10, I hope that you can learn enough Japanese to be a contributor to the Japanese Beta... they still need to incorporate 3 more writing style... maybe you can help!