I think that entries for mnemonics should be always put just under entries
for corresponding text.

JFileChooser uses some mnemonics which are defined in basic*.properties.
LAF-specific properties files have entries for text but don't have entries
for mnemonics for the corresponding text.
(An example)
  In basic.properties:
     FileChooser.openButtonText=Open
     FileChooser.openButtonMnemonic=79

  In motif.properties:
     FileChooser.openButtonText=OK

  Under Motif LAF, in en localle, the "OK" button has a mnemonic 'O'.


Case 1:
  L10N engineers for some languages(e.g. Japanese) have to change the
  way of translation of text with consideration for whether a mnemonic
  exists for the text. If a mnemonic exists, they have to translate the
  text to
     FileChooser.openButtonText=JJJJJ (C)
  If it doesn't exist, they have to translate the text to
     FileChooser.openButtonText=JJJJJ

Case 2:
  In basic_it.properties:
    FileChooser.openButtonText=Apri
    FileChooser.openButtonMnemonic=80
  IN motif_it.properties:
    FileChooser.openButtonText=OK

  JFileChooser under Motif LAF has the "OK" button both on en locale and
  on it locale, but the mnemonic 'O' cannot be used on it locale.

---------

Translators usually translate only data which exists in root .properties file.
Due to this bug, they cannot notice that buttons(Cancel, OK, Update, Help)
have mnemonics under Motif LAF and they may have to localize them because
entries for mnemonics don't exist in motif.properties now.
As a result, some mnemonics are useless on some locales now.

As far as I checked, .properties files for GTK LAF and Windows LAF have
the same problem.