Index: trunk/src/org/openstreetmap/josm/tools/I18n.java
===================================================================
--- trunk/src/org/openstreetmap/josm/tools/I18n.java	(revision 4500)
+++ trunk/src/org/openstreetmap/josm/tools/I18n.java	(revision 4501)
@@ -492,7 +492,11 @@
         }
         /* file format:
+           Files are always a group. English file and translated file must provide identical datasets.
+
            for all single strings:
            {
              unsigned short (2 byte) stringlength
+               - length 0 indicates missing translation
+               - length 0xFFFE indicates translation equal to original, but otherwise is equal to length 0
              string
            }