ldm 3.4.4-1

The greeter says so when the language it was asked for cannot be had.

setlocale(LC_ALL, "") returns NULL when LANG names a locale the image does not carry, and leaves the process in C - where glibc also ignores LANGUAGE, deliberately, with C.UTF-8 counting as C for that purpose. Every string then stays in English and nothing anywhere says why. Somebody who has just written LANG into lts.conf and rebooted a client deserves better than an unchanged screen:

locale "de_DE.UTF-8" is not generated in this image; staying in "C"
and showing English. Build the image with ltsp-build-client --locale
to include it.

and for the other case:

LANGUAGE is set but the locale is "C", and glibc ignores LANGUAGE in
the C locale. The image needs one real locale - any one - for LANGUAGE
to take effect.

Both at log level 3, so they appear without LDM_LOGLEVEL being raised. See ltsp 6.3.14-1, whose --locale puts the locales into the image in the first place.