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I introduced initfsencoding() in #8610 to ensure that Py_FileSystemEncoding is not more NULL. In the discussion, Marc Lemburg noticed that falling back the UTF-8 on nl_langinfo(CODESET) error is a bad idea: ASCII is better (I agree).
We cannot fall back to ASCII yet because there are two other problems that have to be fixed before that:
 - Python3 doesn't support surrogates in module filenames: see #8611
 - If Py_FileSystemEncoding is NULL, encoding functions fallback to utf-8 (PyUnicode_GetDefaultEncoding()). #8715 proposes a new PyUnicode_EncodeFSDefault() function to fix this problem
Attached patch is a partial fix for this issue.

PyUnicode_AsEncodedString() contains a special path for the file system encoding. I don't think that it is still needed, but I don't know how to check that. => read msg105810 

Version 2:
 - #8715 has been commited: patch PyUnicode_EncodeFSDefault()
 - fix the documentation according the changes

I tried the patch on my import_unicode branch and it doesn't work if the locale encoding is not ASCII (as the current code doesn't work if the locale encoding is not UTF-8, #8611).
If Py_FileSystemUnicodeEncoding is NULL: PyUnicode_EncodeFSDefault() should use mbcstowcs() and PyUnicode_DecodeFSDefault() should use wcstombcs(). They may reuse _Py_wchar2char() and _Py_char2wchar().
"ascii" should be used in initfsencoding().

initfsencoding() now raises a fatal error on get_codeset() error. Use a encoding different than the locale encoding on get_codeset() only leads to mojibake and encoding issues, it's not a good idea. Close this issue as invalid.