WritePad update: I found out by accident that I could change the language, simply by deleting the app and reinstalling it. It asked me for my main language again and I selected US English. And it still doesn't work for me. I suppose the problem is not the language, it's the handwriting. The app is optimized for American handwriting, mine is obviously too exotic for it. Here's a screenshot to prove it.
I asked for a refund on the iTunes store but I am not very hopeful of getting it, so I may be stuck with this app anyway. Oh well, it's only 8 euros after all.
ETA: LOL, I had cropped the screenshot on the iPad but it didn't really crop the image and I uploaded it whole. Now you can see all the nonsense I wrote. Or tried to write.