[Choose only letters and numbers, this will be sufficient as the prompt on bootup is only in US-English!]
found above Info in the Wiki and i can’t remember if this Info is also shown on the live-system, if user decided to crypt-install !?
if not, it’ll be good for the next release to show this in big red letters on the specific slide.
if yes, my apologies for the noise 
Yes this is an issue for a very long time…
But i never come to the idea to have a warning inside installer this is indeed a good point!
I can fill an enhancement request at calamares upstream, as i have no idea how to put such there.
We could have a warning in welcome but you know… no one will read it there, so best option would be to have it directly on the slide where you select to encrypt for both options automatic partiton and inside manual partition slide.
Should look like this:
I do create a feature request:
sorry @InDisTincT to not use caps lock and huge red letters 
your example is satisfactory and thank you for creating a request.
and after first successful boot into the system, there should be an additional Info, maybe on the welcome pad, that the user should consider adding more keys (passwords) with e.g. cryptsetup luksAddKey /dev/nvme0n1p2, because it’ll help those who’s maybe confused one day, which password was used and the fact that the keyboard is only EN could confuse the user more at that moment, especially for all non EN users.