Where can I find it?
Some time ago, I could see how many Xp are needed to level up on a panel on the right, but now it's gone. Is it still here, if yes, how can I find it?
That is part of an A/B test. I also had this, but it reverted back to the flags (eternally grateful, I am). Anyway, now the only way (for those of us without that XP panel) is to open the app, either on iOS and Android, and check the XP count from there.
Edit: You can also search for your XP within the code of the page (Inspect Element on Safari), just type in the language that you want to see the XP for, then scrolling down in the sidebar, you'll be able to see the string that holds your XP information.