The properties files for alternate language have less properties defined that the ones for default (master) language.

For example, in atlassian-jira-enterprise-3.0-beta-standalone/atlassian-jira/WEB-INF/lib, the file com/atlassian/jira/web/action/user/ChangePassword.properties (from language_default.jar) have the following content:

changepassword.desc = Enter your new password details below.
changepassword.current = Current Password
changepassword.new = New Password
changepassword.confirm = Confirm Password

changepassword.current.password.incorrect = The current password specified is incorrect.
changepassword.could.not.find.user = Could not get current user to authenticate.
changepassword.new.password.required = The new password cannot be blank.
changepassword.new.password.confirmation.does.not.match = The password and confirmation do not match.

while the file com/atlassian/jira/web/action/user/ChangePassword_fr.properties (from language_fr_FR.jar) have the following:

changepassword.desc = Saisissez votre nouveau mot de passe ci-dessous.
changepassword.current = Mot de Passe Courant
changepassword.new = Nouveau Mot de Passe
changepassword.confirm = Confirmer Nouveau Mot de Passe

Because of this, many strings are not yet translated (in french at least).

Do you have a script that compares the property keys defined in language_default.jar and adds the ones missing in language_*.jar into them ?