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.