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(1) I read "locale setting not supported" and understood (not 
being a native speaker) "(any) locale setting not supported", 
whereas "unknown/unsupported locale" would make it clear 
you can set a locale but not the one that was requested. 
 
(2) I would expect setlocale(..., "de") and setlocale(..., 
("de", None)) to either both fail or both succeed, but 
currently only the latter form works; thus the proposed 
change in the code. 
 
I hope this clarifies the issue and you will reconsider.