Ok, now (I think) I know what's going on. I once accidentally added all possible locales to my Easy Debian images, which took a lot of space. So I deleted most of them and only kept de_DE.UTF-8 (UTF-8 is the default encoding in Linux and therefore Easy Debian) and the en and us locales for fallback reasons.
Maemo however uses ISO-8859 locales (e.g. de_DE). Since my Easy Debian image no longer has the ISO-8859 locale any locale-dependent application that gets the Maemo locale throws warnings. I have them too but since it's not critical I simply didn't care for it. I just installed de_DE in Easy Debian via dpkg-reconfigure (but kept the UTF-8 locale as the default) and the warnings are gone.

So to get rid of your warnings look up what your Maemo locale looks like and install it in Easy Debian. For Estel that would mean Maemo most likely has pl_PL. If he uses my Easy Debian image he doesn't have that locale so he should select that in dpkg-reconfigure. While doing that it might also be useful to add pl_PL.UTF-8 as the Easy Debian's default locale.