I've made progress, but problems don't end:
I've done the whole process again, updating grub for good measure. it told me
there was something wrong with the config: GRUB_TIMEOUT wasn't null (was at
10), which is not allowed anymore if GRUB_HIDDEN_TIMEOUT is defined.
So I've put it back to 0.
I decrypted my drive: the keyboard at this step was probably the default US
one.
But now I land in a BusyBox with (intramfs).
I'll try this:
https://askubuntu.com/questions/137655/boot-drops-to-a-initramfs-prompts-busybox
