It seems that print() and write() (and maybe other of such I/O functions) are relying on sys.getfilesystemencoding(). But these functions are not operating with filenames but with their content. In the attachments is an example script which demonstrates this problem. Here is what I get:

sworddragon@ubuntu:~/tmp$ echo $LANG
de_DE.UTF-8
sworddragon@ubuntu:~/tmp$ python3 test.py
sys.getdefaultencoding(): utf-8
sys.getfilesystemencoding(): utf-8
ä
sworddragon@ubuntu:~/tmp$ LANG=C
sworddragon@ubuntu:~/tmp$ python3 test.py
sys.getdefaultencoding(): utf-8
sys.getfilesystemencoding(): ascii
Traceback (most recent call last):
  File "test.py", line 4, in <module>
    print('\xe4')
UnicodeEncodeError: 'ascii' codec can't encode character '\xe4' in position 0: ordinal not in range(128)