I'm planning on making an open-sourced Chinese version of Java by modifying the open-sourced Java JDK, like how the Easy Programming Language (aka. 易语言) did with Visual Basic. Is this legal?
OpenJDK is GPL-licensed software (GPLv2 with Classpath Exception). You are free to modify this Java implementation, such as by translating the keywords.
However, “Java” is a trademark that you are not allowed to use. You do not have to rename all classes or packages in OpenJDK, but you must not call the resulting language “Java”.
You'll have to read the license and check what it says.
The last time I looked at the license was when Sun owned it, and they explicitly did not allow you to use what they gave you to create incompatabile versions of Java. Probably directly aimed at Microsoft and C#.
Apart from checking whether you are allowed to create a modified language, you need then to check what conditions apply to distributing it.