最新のCVSのCygwinのnew-features.sgmlに次のようなことが書いてあって、
つまりLANG=ja_JPの時はEUC-JPになるらしい。LANG=ja_JP.UTF-8と書けばもちろんUTF-8になるけど。
What's new and what changed from 1.7.1 to 1.7.2
* Localization support has been much improved.
- Cygwin now handles locales using the underlying Windows locale support. The locale must exists in Windows to be recognized.
- New tool "getlocale" to fetch valid locale values from Windows.
- Default charset for locales without explicit charset is now choosen from a list of Linux-compatible charsets.
- For instance: en_US -> ISO-8859-1, ja_JP -> EUC-JP, zh_TW -> Big5.
- Support for the @euro locale modifier to switch to the ISO-8859-15 charset.
- Default charset in the "C" or "POSIX" locale has been changed back from UTF-8 to ASCII, to circumvent problems with applications expecting a singlebyte charset in the "C"/"POSIX" locale. Still use UTF-8 internally for filename conversion in this case.
- LC_COLLATE, LC_MONETARY, LC_NUMERIC, and LC_TIME localization is enabled via Windows locale support.
- fnmatch(3) call is now multibyte-aware.
- New strfmon(3) call.