Previously I was reluctant to use JDownloader 2 BETA version. The main reason is that I will see boxes when the files contain Japanese characters. But this problem solved now.

As I am using Arch Linux, the JDownloader 2 is installed at /opt/JDownloader. We have to create two files:

1. /opt/JDownloader/translations/org/jdownloader/gui/translate/GuiTranslation.en_custom.lng
2. /opt/JDownloader/translations/org/appwork/swing/synthetica/LanguageFileSetup.en_custom.lng

The GuiTranslation.en_custom.lng is a copy of GuiTranslation.en.lng. So that the interface is still using English language. Then LanguageFileSetup.en_custom.lng is a copy of LanguageFileSetup.ja.lng. So that the font is using Japanese language font instead.

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.