When downloading the files contain Chinese or Japanese characters using JDownloader in Linux (for my case, Arch Linux), I found that all the Chinese or Japanese characters are shown with boxes. There are a lot of suggested solutions I found from Internet, especially adding !font=Dialog to the language file. I also tried to edit the fontconfig.properties file, adding path to JAVA_FONTS, etc. But all the solutions do not work.
So, my solution is, just change the user interface style to non-Synthetica style, eg, Substance or Light. Both types of style work well, allow me to see the file names with correct characters. Problem solved.
Please note that, this is not the fault of Synthetica (Synthetica FAQ).