Create output strictly according to the specification for .properties files. This will use escaped Unicode for any non-assci characters. Thus the following string found in a PO file:
ṽḁḽṻḝ
Will appear as follows in the output .properties file:
\u1E7D\u1E01\u1E3D\u1E7B\u1E1D
Mozilla has made slight adjustments to the Java .properties spec. Mozilla will accept UTF-8 encoded strings in the property file and thus does not need escaped Unicode. Thus the above string - ṽḁḽṻḝ - will not be escaped. Mozilla property files are thus more useful for non-Latin languages in that they are actually readable.
Of course this style of file is only used by Mozilla and should not be used for other projects that follow the Java spec more strictly.