Well those locale things are always mysterious to me.
Some stuff on the internet points to running export LC_ALL=en_US.UTF-8 in your terminal, then these should work.
If you want this to be done automatically, you can add this line at the end of your .bashrc file in your user (so e.g. if you’re connected with admin, edit the file with nano /home/admin/.bashrc, add the line at the end of the file, then save/close with Ctrl+X)
About the root cause, well, as said those things are always a bit mysterieous
Depending on how your system was setup, sometimes those things are configured with default, and sometimes they aren’t … So maybe it’s just related to your hardware/VPS setup, idk… (e.g. maybe your VPS provider doesnt configure locales by default somehow)
I would like to mention that I came across the same issue.
For me, it happens some relatively long times after the yunohost installation. It was working properly without any trouble on a raspberry pi 3.
I think this issue arrive after some “working” or “in progress” apps, but unfortunately I was not aware of this soon enough to be more precise.