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When a .properties file is about to be opened and its content cannot be decoded with the current encoding, show a warning dialog and allow the user to choose a different encoding. See http://java.sun.com/j2se/1.5.0/docs/api/java/nio/charset/class-use/CharacterCodingException.html
Now that issue #32392 ("Edit Text Rather than Escape Sequences") is fixed, everything is encoded using a pure ASCII characters and \uxxxx sequences - see also issue #75906 ("Add support for other encodings (other than ISO-8859-1)").