Hi Christophe, and welcome!
These are great questions, thanks for posting them to the DIY Forums for others to see. I'll answer them below:
1) You can absolutely use DIY on the web (without an iOS device). Some Club Leaders have found it easier to utilize DIY through the app, but many more have been using DIY.org. We have talked about building tools for Android, but because we're a pretty small team we won't be able to work on that any time soon.
2) Have you checked out
n00b? This Skill is full of Scratch challenges. I don't think that we would plan a Scratch specific Skill on DIY. However, we love the program, and try to incorporate it wherever it's appropriate.
3) We do have other language options for the DIY app, unfortunately that's only for iOS.
4) If you can do it in Scratch, then perhaps we should change it to "Create a Sprite Sheet". Let me know how it goes. Our Skills team is always open to suggestions.