java.util.ReourseBundle#getBundle methods returns undesirable bundle
if caller( of getBundle)'s default locale(or the locale specified as
an 2nd argument of getBundle) is "zh_TW", the resource for "zh_TW"
is not available, and resource for "zh" is available.
It returns resource for "zh" locale, but it should return default
bundle.

for example, if the resource file in concern is abc.properties, and the
caller's locale is "zh_TW", then

  abc.properties -------- (1)
  abc_zh.properties ----- (2)
  abc_zh_TW.properties -- (3)

ResourceBundle#getBundle should try to find (3) first, then if it fails,
it should not try to find (2) but just (1).