java propertie files should be: ISO 8859-1
I actually was going around in circles with this recently, as I was not aware that .properties files were required to be latin-1. I had sent my solution's i18n to a translation service, and got back a UTF-8 encoded file. When I brought this into Servoy, the first time I opened the i18n editor and saved/updated/added, my high-ascii (accented latin in my case) was changed to unicode literals. This was probably a result of Servoy implementing some version of native2ascii? The trouble I ran into though was that b/c the file was not latin-1, the native2ascii conversion of the high ascii chars to unicode didn't work properly, and I ended up with a completely defunkt .properties file. What I ended up having to do was run native2ascii at the command line on the utf-8 file and pass in a flag that told it that the file was actually utf-8. When I did this the high ascii was escaped properly and then when I brought it into servoy all was well.
I think that in order to prevent other's from learning all of this the hard way as I did, perhaps Servoy can run a charset detect on the .properties file and pass the gotten encoding as a flag to its native2ascii implementation? This will make it somewhat dummy proof.
Just an FYI/sharing my experience